Full text of "The easy guide to your Atari 600XL/800XL"

See other formats

SYBEXCOMPUTER BOOKS

THE EASY GUIDE TO YOUR

600XL/
800XL

''■^*,

''<?<%.,,

^^'^^^^i^--

THOMAS BLAC

C.id""?' J

THE EASY GUIDE
TO YOUR ATARI 600XL/800XL

THE EASY GUIDE

TO YOUR ATARreooxr/Booxr

Thomas Blackadar

BERKELEY • PARIS • DUSSELDORF

Cover art by Daniel Le Noury
Book design by Mary Rose Ogren

A*^' ^'^ *?"' ^'T *'"• ^'*" ^^L> A'^ 800, Atari 800XL, Atari 810, Atari 1010
^i^Avi^ 'a 'w '""^ Atari 1027, Atari 1050, Atari 1200XL, Atari 1400XL, Atari
1450XLD, AtanWnter, Centipede, MissUe Command, and Star Raiders are all trademarks of
Atan, Inc.

Choplifter! and Serpentine are trademarks of Broderbund Software Inc

Facemaker, Kindercomp, and Snooper Troops are trademarks of Spinnaker Software, Inc

Fac-Man is a trademark of the Midway Manufacturing Company

Shamus is a trademark of Synapse Software, Inc.

VisiCalc is a trademark of VisiCorp.

Zork is a trademark of Infocom, Inc.

SYBEX is not affiliated with any manufacturer.

Every efiFort has been made to supply complete and accurate information. However SYBEX
^umes no responsibUity for its use, nor for any infringements of patents or other rights of
third parties which would result. Manufacturers reserve the right to change specifications at
any time without notice. o t~

Copyright©1984 SYBEX Inc. World Rights reserved. No part of this publication may be
stored m a retrieval system, transmitted, or reproduced in any way, including but not limited
to photocopy photograph, magnetic or other record, without the prior agreement and written
permission of the publisher.

Library of Congress Card Number: 84-50365
ISBN 0-89588- 125-X
Printed in the United States of America
10 987654321

ACKNOWLEDGEMENTS

A book such as this is always a joint project, and many people
share the credit. I'd particularly like to thank Rudolph Langer and
Paul Panish for their support and advice in the development of
this project. Joel Kreisman contributed valuable technical assistance
at all stages, and David Kolodney gave it a careful and sensitive
editing. My thanks also to these other wonderful people who did so
much to turn a manuscript into a book: Valerie Robbins and Leslie
Horston, word processing; Gloria Maya, Valerie Brewster, and
Cheryl Vega, typesetting; Janis Lund and Elizabeth Thomas, proof-
reading; and Mary Rose Ogren, design. Their contributions are
much appreciated.

At Atari, Margaret Lasecke and her staff were extremely helpful.
From my initied contacts with Bruce Quill to my final houndings of
Trish Marino, everyone was friendly £ind cooperative.

TABLE OF CONTENTS

INTRODUCTION

SECTION

Getting
Acquainted

MEET THE ATARI COMPUTERS
Fear 3

What is a Computer? 4
What Can it Do? 5
The Atari Computers 6
Setting up Your Computer 8
Peripheral Equipment 14

COMMERCIAL SOFTWARE
How to Buy Software 1 7
Games 19
Serious Programs 22
How to Use Software 24
A Look Ahead 25

THE KEYBOARD AND THE SCREEN
The Keyboard 27
Controlling the Cursor 31
RETURN and the ERROR Message 34
Your First Command: PRINT 36
The ESC Key 40

VIU

The Special Function Keys
Optional Exercises 45
Summary 45

ATARI GRAPHICS
What are Graphics? 47
Text: GRAPHICS 48
The POSITION Statement 49
Four-color Drawing: GRAPHICS 7
Optional Exercises 59
Summary 59

SECTION

Programming

WRITING A PROGRAM 63

Storing Your Program 64
The Box-Drawing Program 70
Hints on Writing Programs 71
Debugging 72
Cassette Storage 75
Optional Exercises 76
Summary 77

USING VARIABLES 79

Storing Numbers 79
Variables in Programs 83
The READ and DATA Statements 88
INPUT: Getting Values From the
Keyboard 90
Doing Calculations 94
Strings: Storing Letters 97
Optional Exercises 99
Summary 99

CONTROLLING YOUR PROGRAM 1 01

GOTO: jumping to Another Statement 101
FOR/NEXT Loops 106
IF Statements: Making Decisions 1 1 3
COSUB: Subprograms 1 1 7
Optional Exercises 1 20
Summary 120

ADVANCED GRAPHICS 1 23

Changing Colors with SETCOLOR 1 23
Eliminating the Text Window 129
The Graphics Modes 1 30
The Text Modes 1 35
Summary 139

SECTION

Storage

CASSETTE STORAGE
Setting Up the Program Recorder 144
Loading Prerecorded Cassette
Programs 145

Saving Your Own Programs 146
Other Ways to Save Programs 1 49
Summary 151

143

10 THE DISK DRIVE 153

Setting Up the Disk Drive 1 54
Taking Care of Diskettes 1 56
Turning on the Disk Drive 157
Using Prerecorded Diskette Programs 161
The DOS Diskette 162
Files 165
Saving Your Own Programs 1 67

Duplicating Diskettes 168
Summary 170

1 1 AFTERWORD

Other Add-ons 1 71
Other Directions 173

171

Appendices

A FURTHER READING

B REFERENCE GUIDE TO BASIC

C ATARI ERROR CODES

177
179
187

ANSWERS TO SELECTED EXERCISES
INDEX

191
195

INTRODUCTION

If you have recently bought an Atari® computer, you may have
been surprised when you opened the box. The machine was there all
right, but what about the instruction book? All you could find was a
thin Owner's Guide and an Atari BASIC Reference Guide, forbiddingly
marked "for experienced programmers only."

I'm sorry to have to say it, but that's all there is. Atari freely
sidmits that it does not try to teach more than the essentials of setting
up the machines. "If you want more information," they say, "you
should buy a book."

Your Atari computer is a wonderful machine: it works well and can
grow with your needs. I have written this book to be your guide, to
help you learn quickly what you need to know about your computer.

In the course of this book, you will set up your computer and begin
to use it. You will find out where you can buy programs that will let
you start using your computer right away. Then you will learn how to
write your own programs, and how to store them on a program
recorder or disk drive.

What do you need to know beforehand? Nothing. This is an easy
guide, which zissumes you have never used a computer before. You can
stzirt from the very beginning, and move slowly enough to master the
concepts without needing to cope with computer jargon.

If you know a little alrezidy, you can still use this book. You may
want to skim through sections that you already understzuid, then read
more carefiilly once you get to the sections you still need to cover.

I have tried to make this book both serious and enjoyable. I have
assumed that you zire truly interested in learning about your Atari
computer, and that you wzmt a clear, no-nonsense introduction. At
the same time, I have concentrated on the most attractive, entertain-
ing features of your Atari.

The chapters are grouped into three sections. This wUl help you
organize your explorations into three fundamental phases: Getting
Acquainted, Programming, and Storage.

The first section, Getting Acquainted, will help you learn the fun-
damentals of your machine. Before you can do anything else, you
must learn how the computer works and how to type simple com-
mands. By playing around with the machine, you will become
comfortable using it.

Section Two moves into programming. You can combine simple
commands into a larger plan in order to perform more complex tasks.
As you learn the concepts of computer programming, you will also be
learning BASIC, the primary language used by Atari computers and
one of the computer languages most commonly used today.

Section Three addresses a subject treated only briefly earlier: how
to store your programs permanendy on tapes or diskettes. Chapter 9
describes how to use the Atari Program Recorder with cassette tapes.
Chapter 10 shows how to use a disk drive for faster, more reliable
storage. At some points in this book, you may want to glance ahead
to these chapters, if you want to use commercial programs or store
your own programs for future reference.

To use this book most effectively, you will need to have access to an
Atari computer For most of the book, you should have your com-
puter sitting in front of you, so that you can try out die examples as
you read.

It doesn't matter which Atari computer you have. This book is
directed specificaUy at the new Atari 600XL™ and SOOXLTm, but the
descriptions will also work for the other Atari models. The projected
Atari 1400XLTM and 1450XLD™ will both be essentiaUy die same
machine as the 800XL, with some additional features. Everything
said in this book will also apply to them, and any differences will be
noted in the text.

If you have one of the older Ataris (the discontinued 4001"^^ 800^^,
and 1200XL™ models), you wiU need Atari's BASIC Comptlting Lan-
guage cartridge to use this book. If you plug this cartridge in, your
computer will behave just like an Atari 600XL or 800XL, and you
can run any program just as if you had one of the new machines.
There may, however, be a few differences in the detaUs about the key-
board and graphics. Read pages 6-8 of Chapter 1 if you are thinking
of using this book with a model other than the 600XL or 800XL.

Computers may seem mysterious while you are an outsider, but
after your first steps, you will be able to continue on your own. This
book merely aims to make those first steps a little easier.

Section

Getting
Acquainted

B CHAPTER ONE ; —

Meet the
Atari Computers

You have recently bought an Atari computer, and are anxious to
start using it. You are curious to find out what your machine can do,
jmd how you can control it.

This chapter is an overview. It gives some tips that will help you
become comfortable with your Atari, and detzuled instructions on how
to set it up. By the end of the chapter, you will be ready to use your
computer.

FEAR—

Before you begin your journey, you'll need to overcome your fears.
If this is your first time on a computer, you are probably a little intimi-
dated by it. You don't have to be: a computer is a machine, nothing
more. Like a television set or a dishwasher, a computer can help you if
you learn how to use it, and it won't bite you if you make a mistake.

You don't need to be afraid that you might wreck your computer
by pressing the wrong key. Your computer is an electronic device that
deserves the same care you give to a radio or a pocket calculator, but
you don't have to be timid with it. There is litde you can do in nor-
mal use that will harm it. At the very worst, you might lose the words
and numbers you have just typed, but you won't hurt the machine.

Your AtJiri gives you a kind of panic button. This is the RESET
key, located along the right side of the keyboard. If you ever get lost
or don't know what's happening, you can always press this key. The
computer will go back to the way it was when you turned it on, ready
to start again. And if that doesn't do the trick, you can always turn
the power oflF and back on.

4 THE EASY GUIDE TO YOUR ATARI 600XL/800X

If you have some lingering fears, play a game. There is no better
way to become comfortable with a computer than to have fan with it
In Chapter 2 of this book, you will find out how to buy some of the
best Atari games, but for now, you might want to start with the most
famous: Pac-Man. Atari's home version of this game is truly
excellent— a sure-fire cure for computer fear.

WHAT IS A COMPUTER? .

How does a computer work? You don't need to know. You can
think of It as a dosed box that takes in information, chums it around
and spits out die results. Along the way, it must do many complex
operations, but diat's not something you need to concern yourself
with. Just think about what goes in and what comes out.

You talk to the computer through its keyboard. Atari computers have a
M set of typewriter keys that let you type letters, numbers, and special
symbols. You wiU use these keys for all your contact with the computer.

To talk back, die computer has an electronic circuit that can display
words and pictures on your television screen. So, as you type letters
on the keyboard, your computer shows them on the screen. Then, if
It needs to give an answer, it displays that as well. With graphics, you
can also have the computer draw pictures on the screen.

The computer has a memoiy, so diat you can store information for
fiiture reference. You can use this memory to save the lines of a pro-
gram, the numbers you will use in a fiiture calculation, or the words
of a text you are writing.

How much you can store at any one time depends on the size of
your computer's memory, which is measured in bytes. A byte is
roughly die space needed to store a single letter or a three-digit num-
ber. You will often see memory listed widi letter K, which means
roughly a thousand bytes (1024 to be exact). One K would hold about
one third of this page.

For the purposes of this book, you won't need to worry about the
size of your computer's memory. The Atari 600XL has 16K bytes of
memory built in— more than enough for anything in this book. The
other Ataris have even more. Later on, you may want to buy a mem-
ory expansion for the 600XL so that you can run more complicated
programs.

One other thing about your computer's memory: everything you

MEET THE ATARI COMPUTERS

Store in it is erased when you turn the power off. To store information
permanently, you must attach a program recorder or a disk drive.

WHAT CAN IT DO?

Your Atairi computer can perform complex calculations quickly and
easHy, display bar graphs, and paiint colorful pictures on your televi-
sion screen. With a word-processing program, it can let you revise a
letter without retyping it. And of course, it can play games.

The computer's great strength is its flexibility. It can do numerical
calculations if you wish, or limit itself to letters and words. It can
react to the keyboard and arrange the screen display in any way you
wish. With its memory, it can store and arrange information in a vari-
ety of useful ways.

You control your computer with a program, a detailed list of instruc-
tions that it follows like a recipe, step-by-step, to perform its task. The
program can be as simple or as complicated as you like, and can ask
the computer to make decisions as it works. Programs are often called
software.

A good way to start is to buy programs that other people have writ-
ten. When you use commercial software, you don't need to know how
it works. You just give general instructions and zmswer a few ques-
tions, then let the program take care of the detzdls. Chapter 2 of this
book is an introduction to some of the software that is conunercially
available. Games are very popuku-, and the Atari computers have by
far the best selection of entertainment software of any home computer.
You can, however, also buy programs for mainy other purposes, such
as education and home management. Word processing has become
increjisingly popular junong those who vfant the convenience of an
electronic typewriter.

At some point, however, you may v/ant to write your own pro-
grams, to gain direct control over your computer. Commercial
programs are expensive and sometimes inconvenient. If you plan to
use your computer to its fuU potentizd, you will want direct control.
In the later chapters of this book, you will lezun how to write your
own programs. This will let you tell your computer exactly what you
wcint it to do. At the same time, it wiU let you explore many of the
computer's deeper capabilities, so that you will be the master of
your machine.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

THE ATARI COMPUTERS

Atari has sold many different computers since it entered the busi-
ness. To avoid confusion, I'd hke to explain their history and their
minor differences. Figure 1.1 is a comparison chart of the models.
They can be divided into two groups: new and old.

The new ones, sold starting late in 1983, all contain XL in their
model numbers and feature a sharp-looking white-and-brown case.
Of these new machines, two were available as this book was going to
press: the low-priced 600XL and 800XL. Two others, the 1400XL
and 1450XLD, were still under development. Rumor has it that one
or even both of these may be cancelled, or never even introduced.
They are mentioned in this book for the sake of completeness, but are
not covered in detail.

Before Atari introduced these new machines, it had been selling two

FEATURES

600XL 800XL
(available)

1400XL 1450XLD
(planned)

400 800 1200XL
(discontinued)

Memory size

16K 64K

64K 64K

16K 48K 64K

Typewriter-styie
keyboard

• •

* • •

XL-style design

• •

•

Programmable
function keys

•

Built-in BASIC

• •

Monitor

connection

•

• •

Telephone

connection

Speech
synthesizer

Built-in disk

drive

Graphics modes

n Ti

n 11

6-9 6-9 11

Text modes

5 5

3 3 5

*flat keyboard

Figure I.J; The features of the Atari computers.

MEET THE ATARI COMPUTERS

Others: the Atari 400 and the 800. In between, there was the Atari
1200XL, an intermediate model that was sold for only a few months.

Fortunately, all of these models are just variations on the same
machine. The older models lack a few features, and require that you
plug in a separate BASIC language cartridge. Some of the new XL
computers have extras built in, but for the most part the machines are
functionally identical, and all run the same prograims.

This book is directed primarily at the Atari 600XL, shown in Figure
1.2, and at the 800XL, a lairger version with more memory. However,
you can also use this book with any of Atari's other computers, old or
new. All of the programs described here will work on the new
machines, and the physical descriptions should vziry only in minor
details. If you own one of the older computers (400, 800, or 1200XL),
you will need to make only a few smaJl allowainces.

Why all these different models? Atziri has tried to design a line of
computers that will appeal to everyone. It sells the Atari 600XL for
those who want merely the bzisic machine for games and simple pro-
gramming. For those who want a more powerful machine, it sells two
higher-priced models, each of which adds features to the one before.
The machines are so similar, however, that you can make the 600XL
into any of the others merely by adding external devices.

If you add a memory-expansion module to an Atari 600XL, it
effectively becomes an 800XL. The future 1400XL and 1450XLD
will be the same as the 600XL and 800XL, but with some extra

Figure 1.2: This book is aimed at the Atari 600XL. The 800XL is essentially the
same, but with additional memory.

8 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

features. They wiU contain a telephone modem, which will aUow them to
transmit programs and information to other computers over the tele-
phone. They wiU also include a speech synthesizer, which lets the computer
pronounce English words out loud. The 1450XLD will have, in addi-
tion, a buUt-in disk drive and a space for a second one. If yoii buy one
of these machines, you can initially learn to use it just as if it were a
600XL, and dien move on to master its additional features.

If you have one of the Atari 400, 800, or 1200XL models that were
sold before the end of 1983, you can still use this book, but you wiU
have to be more careful. To begin widi, the physical design of the
machme is different from the new models, and some of the keys have
been changed. As you read about the keyboard in Chapter 3, you will
need to adjust some of the details to fit your machine.

Another difference involves graphics. WhUe the basic graphic-
display systems are the same in both the old and new Ataris, die XL
versions have several additional graphics modes. Some of these are
discussed in Chapter 8. If you have one of die older machines, you
wiU not be able to use these advanced graphics features, but you will
have no problems with the rest of this book.

Unlike the new models, the discontinued Atari 400, 800, and
1200XL do not have the BASIC programming language buUt in. If
you want to do anything more than run commercial software you
will have to buy Atari's BASIC Computing Language cartridge. To
use this book with one of the older computers, you must have this BASIC cartridge
plugged in. Figure 1.3 lists the Atari computers and describes what
allowances, if any, you will need to make as you read on.

SETTING UP YOUR COMPUTER

The only thing you'U need tiiat does not come with your computer
IS a television set. You can use any normal television, but you will get
the best results from newer models. Televisions more dian ten years
old may give a jittery screen image.

A color television is strongly recommended. The Atari computers
have a spectacular color graphics system that can display 256 different
shades. Many of the programs in this book take advantage of diese
displays. WhUe you can run them on a black-and-white television,
you will be missing a lot.

If you are selecting a television to use with your computer, you

MEET THE ATARI COMPUTERS

Atari 600XL

Atari 800XL

This book was written specifically for this
machine. No special adjustments are necessary.

Atari 1400XL
(projected)

Atari 1450XLD
(projected)

Atari 400
(discontinued)

Atari 800
(discontinued)

Atari 1200XL
(discontinued)

This machine is essentially an Atari 600XL with
extra memory You can use this book as if you
had a 600XL.

if it becomes available, this machine will be
the same as the Atari 800XL, except that it will
have a larger keyboard and some added fea-
tures. You can use this book, but it will not
describe the built-in telephone modem or the
speech synthesizer.

Like the 1400XL, this model is still in the plan-
ning stage. When it appears, it will have all of
the same features as the 1400XL, plus a disk
drive. For the basic computer, however, you can
still use this book.

This older machine has the same internal opera-
tion as the Atari 600XL. Its flat keyboard makes it
harder to use than the other Atari computers.
The physical descriptions in Chapter 3, and the
advanced graphics in Chapter 8 will be some-
what different. YOU MUST HAVE AN ATARI
BASIC CARTRIDGE.

This older machine has the same internal opera-
tion as the Atari 800XL. The physical descrip-
tions in Chapter 3, and the advanced graphics in
Chapter 8 will be somewhat different. YOU
MUST HAVE AN ATARI BASIC CARTRIDGE.

This discontinued machine looks the same as
the Atari 1400XL, but does not have the tele-
phone modem, speech synthesizer, or built-in
BASIC. To use it, YOU MUST HAVE AN ATARI
BASIC CARTRIDGE. In all other respects, it
resembles the machines described in this book.

Figure 1.3: How to use this book wilh any Atari computer.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

should probably choose a small or medium-size screen. Screens mea-
suring from 10 to 15 inches are quite large enough to be readable.
Larger screens are unnecessary, and often harder to read.

All die Ataris except die 600XL and the discontinued 400 allow
you to hook up a color monitor instead of a television. A monitor looks
like a television, but lacks a tuner and thus cannot be used to receive
normal broadcasts. A monitor wiU usually give a clearer picture than
a normal television. Some monitors also give you higher resolution
(finer detail), so that with some additional equipment you can display
more than 40 characters per line.

When you open the box, you will find an owner's manual and sev-
eral pieces of equipment. Unpack the box carefiilly, and make sure
you don't lose anything. You should find three major items.

First, there is the computer itself. Set this on the table where you
will be working and turn it around so that you can see the back, as
shown in Figure 1.4. Models other dian die 600XL will look slighdy
different.

The second item in die box is a power supply, which converts your
house's electricity into the direct current used by your computer. This
box has a power cord and a cable diat you plug into the back of die
computer.

The last items you will find are a short cable and a small switch box
that you will use to connect the computer to your television. The
cable and switch box come in several forms, but the most common is
shown in Figure 1.5. The box has a small black pin diat you can slide
between two positions. When it is in the COMPUTER position, your

Figure 1.4: The back of the Atari 600XL, where you will make your connections.

MEET THE ATARI COMPUTERS

television will display the computer's images. Slide the pin back into
the TV position to watch normal prograims.

Once you've checked that you have everything, you can connect
your computer. You can easily do this yourself with nothing more
than a screwdriver.

Before you start, make sure your television and computer are
turned off. None of the cables is dangerous to touch, but it is a good
practice always to turn the power off before making cormections.

Don't be confused by the label POWER at the lower-right comer
of the Atari 600XL amd 800XL keyboard. This label refers to the red
indicator light that shows when the power is on. The actUEil on/off
switch is located on the back of the computer, at the left end. On the
1200XL, 1400XL, and 1450XLD, it is along the left side of die com-
puter, next to the cartridge slot. On the 400 and 800, the switch is on
the right side of the machine. The switch is always mzirked POWER
or PWR ON/OFE

Start by connecting the switch box to your television. Disconnect

Figun 1.5: The switch box.

12 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the antenna or rabbit ears. On most sets, the antenna is connected
with a double cable to two screws on the back of the set labeled VHF.
Loosen these screws and remove the wires. You don't need to discon-
nect the antenna labeled UHF. (If you are connected to a cable TV
system, see below.)

The switch box has a pair of short wires coming out of one end.
Connect these wires to the two screws from which you just discon-
nected the cinterma. Tighten the screws.

On the side of the switch box labeled TV or ANTENNA, you will
find another pair of screws. If you plan to use your set for normal
television as well as for your computer display, attach the wires from
your antenna to these screws. When the switch is set to the TV posi-
tion, your antenna will be connected through the switch box just as if
you had never removed it from the set.

If you subscribe to a cable network, your television is probably con-
nected with a single round plug instead of a pair of screws. Some
Atari switchboxes do have a large round connector next to the two
screws; you can use this if it fits the plug on the end of your cable. If it
does not fit, you will have to buy an adapter that connects to the
screws. You can find these in most television or electronics stores (try
Radio Shack).

Find the black cable that came with the computer. It should
be about six feet long cind have a round metal plug on each end. Con-
nect one end to the hole in the side of the switch box labeled
COMPUTER. Then connect the other end to the hole labeled
SWITCH BOX on the computer itself. On all the new XL models,
this hole is to the right of center as you face the back.

Now plug in the power supply. On all Atari XL computers, the
power supply plugs into the hole labeled PWR.IN, which is located at
the right as you face the back. Connect the power supply here, then
plug it into an oudet in your house. Your computer should now be
cormected as in Figure 1.6.

You can have your television receive the computer's signal on
either channel 2 or 3. You will generally get less interference if you
choose the one not used by a television station in your area. Set the
charmel-selector switch on the back of your computer, then tune your
television to the same channel.

If everything is turned on and connected properly, you should see a
blue screen with the word READY in the upper-left corner. If the

MEET THE ATARI COMPUTERS 13

image is fuz2y, adjust the fine tuning on your television. If the screen
isn't dark blue, adjust the color, tint, and brightness controls on your
set. You may find you'll need to turn off the automatic fine tuning
(AFT) or automatic color controls: some televisions provide a better
image when adjusted manually.

By the way, don't worry if the screen color starts chzmging after a
few minutes. Atari computers are designed to do this if you leave
them on without doing anything. This is a useful feature, designed to
protect your television. If the computer did not shift the colors, its let-
ters could become permanendy burned into the screen. The computer
returns to the blue screen when you type any key.

If you can't get a picture or the image will not become dear, you
may have done something wrong. The problem could be any of
the foUowing:

• You have forgotten to turn the computer or the television
on, or have forgotten to plug them in.

• The cable from the computer to the switch box is not con-
nected securely. Try wiggling the connections.

• You have not tightened the wires from the switch box
securely to the VHF screws on the television.

• The switch box is not in the COMPUTER position.

• Your television is not tuned to the same channel (2 or 3)
as your computer. Try changing to the other channel.

Figure 1.6: Connecting the computer to the television.

14 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

If you have checked all of these and your computer still doesn't
work, you should contact your dealer. The machine you have bought
could be defective, or you might be missing something. Your Atari
dealer can give you advice, and can replace defective equipment. If
you can't get satisfactory service from your dealer, call Atari's
customer service hotline (800-538-8543; in California 800-672-1404).

PERIPHERAL EQUIPMENT

With a television set and the equipment that came with your com-
puter, you have everything you'll need to use this book. You can run
your computer, write BASIC programs, and use cartridge programs
sold by other companies.

There are, however, a variety of devices you can attach to your
computer to expand its capabilities. None of these peripherals is essen-
tial, but some will make your life easier.

If you are planning to play games, you will probably want to buy a
joystick. It has a handle that you can move in various directions to con-
trol figures on the screen. It also has a small red fire button, used in
some video games to fire shots. Your Atari lets you connect a second
joystick for games in which two people play at the same time.

You may want to use the Atari Program Recorder with your com-
puter. This device lets you save programs and data permanendy on a
standard cassette tape, so that they are not lost when the computer is
turned off. Later on, you can load them back into the computer for
reuse. The program recorder also lets you use games and programs
that are sold as prerecorded cassettes.

Cassette tapes, unfortunately, tend to be slow and cumbersome. If
you are doing a lot of serious work, you might want to buy a disk drive
instead, which lets you store and retrieve large quantities of informa-
tion very rapidly. A program that takes several minutes to load from a
cassette might require only a few seconds with a disk drive. Also,
some advanced programs are sold only for a disk drive.

The cassette recorder and disk drive are described in diis book, but
not until Chapters 9 and 10. There may be times earlier in the book
when you will want to use these devices if you have them— to save a
long program, for instance. At these points, the text will suggest that
you glance ahead to Section Three if you want to use this extra equip-
ment. In diis way, you can learn to use the cassette recorder or disk

MEET THE ATARI COMPUTERS

drive if you have one, but you will not be distracted if you don't.

You may also want to add a printer to your system. As its name sug-
gests, this lets your computer print numbers zmd letters on paper.
This is especially useful if you want a paper copy of programs you are
writing, or if you want to do word processing.

Atari makes several printers of dififerent kinds and qualities. The
cheapest is the 1020 printer/plotter, which displays text and graphics
on a 4-inch-wide tape. For a litde more money, you can buy a printer
that uses full-size sheets of typing paper. One of these, the 1025 dot-
matrix printer, displays letters as an array of small points. While it
prints quickly and legibly, the output is not terribly attractive. For
higher quzdity, you should consider the new 1027 letter-quality printer,
which prints characters that resemble a normal typewriter's.

The program recorder, disk drive, and printer all cormect to the
same large hole in the back of the computer, labeled PERIPHERAL.
If you want to use more than one device at a time, you can chain them
together. Connect a cable between the PERIPHERAL hole on the
computer and one of the holes in the back of the first device. Then
connect a cable between the second device cind the hole you didn't use
on the first. In this way, you can have several different peripherals
connected to your computer. You can connect the devices in any
sequence, except for the program recorder, which must come at the
end of the chain.

One other note about Atari's line of peripherals: When Atari intro-
duced its new XL computers, it also redesigned its entire line of
peripherals. Fortunately, the company has taken great care to make
sure that all of its old and new machines are compatible. All of the old
peripherals will work on the new machines, and aH of the new periph-
erals will work on the old line. Figure 1.7 shows how the model

DEVICE:

OLD MODEL

NEW MODEL

Program Recorder
Disk Drive
Printer (4" tape)
Printer (dot matrix)
Printer (letter quality)

410
810
820
825

1010
1050
1020
1025
1027

Figure 1.7: The old and new Atari peripherals.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

numbers correspond. Check with your dealer if you have any doubts
about compatibility.

We have now reached the end of the preliminaries. You have set up
your machine, and it is waiting to go. With the next chapter, you will
start using it. Good luck, and enjoy yourself.

B CHAPTER TWO

Commercial
Software

In the first chapter, you found out what your Atari computer is,
and what the features of your model are. You found out something
about how the computer works, and learned that you don't need to be
afraid of it. Finally, you have set it up and turned it on. Your screen
says READY, and you are eager to do something.

The best way to begin using your computer is to buy programs
written by other people. In many cases, you can simply plug the pro-
gram into the computer and start it running. You don't need to know
anything about how the computer works, and you need no program-
ming knowledge.

You can buy all sorts of software irom Atari and other mzinufactur-
ers. Games are the most popular, but excellent programs are also
av2iilable in other areas, such as education, home finzince, and music.

This chapter will start by describing the range of what is available.
While this is not a full buyer's guide, it will give you some tips on
good programs. It will also discuss some of the things you should
know as you shop, so that you can protect yourself from the bad
programs that are sold alongside the good.

The second pzirt of this chapter will show you how to use progrzims
you have bought. Most programs come with instruction books that
explain the details of operation, but I can give you some general guid-
ance to help get you going.

HOW TO BUY SOFTWARE

Atari itself sells more than 250 programs for its own computers,
and claims that more than 2,000 are available from other sources.

18 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

From this large selection, you should have no trouble finding some
that suit your needs.

Programs are sold for the Atari computers on cairtridges, cassettes,
or disks. Games tend to be sold ais cartridges, while more serious pro-
grams are usually sold on tape or disk.

A cartridge stores its program in an electronic circuit, which the
computer can read directly. Because they plug directly into the com-
puter and cannot be erased, cartridges are generally more convenient
and more durable than other forms of software.

Prerecorded cassettes are used with the Atairi Prograim Recorder.
With cassettes, the program is encoded as vziriations in a high-pitched
whisde. The computer can decode, store, and use these messages.
Cassette tapes have several important drawbacks. Before you can use
them, you have to load the program into the computer's memory.
This loading procedure can be a little complicated, even if you foDow
the procedure in Chapter 9 of this book. Cassettes are slow, taking as
long as five minutes to load a complex program. They jire also less
durable than czutridges, and can be made unreadable by magnetic
fields or mechanical wear.

A disk drive is more convenient. This device uses 5V4-inch plates
called diskettes covered with a magnetic recording surface that can store
large amounts of information. You still have to load diskette progrjims
into the computer before you use them, but the procedure is faster
and more reliable than with cassette tapes. Most software manufactur-
ers sell their cassette programs in diskette form as well. Diskettes and
disk drives are described in Chapter 10.

You can buy programs for your Atari computer at most places that
sell Atari computers or video-game machines. Department and dis-
count stores usually carry some programs, especially the more popular
games. Many record and television stores now also carry software for
home computers. In large cities, you can usually find stores that sj)e-
cialize in computer software. These are your best bet for finding the
less popular titles.

Atari's own programs are usually very good. Atari deserves its rep-
utation for fine video games, and it has an excellent line of home-
management programs as well. While Atari does occasionally let a
mediocre program slip through, it has very good overall quality. Atari
also runs a service called APX (for Atari Program EXchange), which
sells programs submitted by Atari users. Many of these titles are quite

COMMERCIAL SOFTWARE 19

useful, though they tend to be unpolished. Most APX programs are
sold on diskette.

A number of other companies sell programs that run on Atari com-
puters. Atari has wisely encouraged this practice in the belief that
a wide selection of software will help to sell its machines. These
independent companies are cadled third-party memufacttarers.

If you choose your programs off the shelf, you will learn an
unpleasant fact: mixed in with the truly excellent programs, there aic
many poor ones. In the fast-moving computer business, some com-
panies have been able to sell inferior products merely by wrapping
them up in attractive packages. Even Atari has sometimes been guilty
of rushing programs to the market before they were really ready.
Since you usually have no way to look at the program before you buy
it, you can easily make mistakes.

There axt several resources that can help you. Addison- Wesley
publishes an index of Atari programs called The Book of Atari Software,
for $19.95. While the catalog format is a litde unwieldy, this book
does give short descriptions and ratings of most of the programs
currendy avzdlable.

Magazines are zdso helpful. Compule! is a general monthly magazine
that often reviews new programs oflFered for Atari home computers.
ANTIC, a magazine devoted to the Atari computers, often describes
new products.

GAMES

I would suggest you start with a game if you are using your com-
puter for the first time. You can do more serious work later.

You can buy mzuiy types of games for your Atjui computer. Atari
has made its name with its Izu-ge selection of excellent video games,
many of which became famous in the video zircades. Game programs
usually cost between $25 and $40.

You will generally need to have a joystick to play games. Atari's
joystick costs less than $10, and can be found at any dealer or dis-
count store. If you are a serious g£une player, you might want to
invest in a fancier joystick, for greater comfort and better control. For
certain games, you might also consider an £ut:ade-style trackball, which
lets you control your players by rolling a ball in the direction you
want to move. These cost between $50 and $75.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Of the arcade games available for the Atari computers, Pac-Man is
the most famous. Figure 2.1 shows the home computer version of the
game. Your object is to move your small yellow man around the maze
and eat all of the dots, without being caught by one of the four roving
monsters. You can achieve temporary safety by eating one of the
larger "energizer" dots in the four comers of the screen. This allows
you to chase the monsters and eat them instead of being eaten. After
a short time, however, you must return to your task of eating dots
aind avoiding monsters.

Pac-Man is an exceUent bargain. Atari's home-computer version is
very well designed, and retains all of the important features of the
arcade game. Even if you aren't a video-game fanatic, the Pac-Man
cartridge will give you a fine introduction to some of the things your
computer can do.

Atari sells several other video games for its home computers. Centi-
pede and Missile Command are familiar tides from the video arcade.
In Star Radders, an original Atciri game, you pilot a spaceship in a
batde against a fleet of enemy attackers. You must plan your strategy

Figure 2.1: The Pac-Man game.

COMMERCIAL SOFTWARE

carefully to conserve your fuel and protect your defensive shields. As
you fly, you see a view of the stars aind your enemy's ships moving by.

Of the games made by other companies for Atari computers, my
personal favorite is Serpentine, by Br^derbund Software. In this
game, shown in Figure 2.2, you maneuver a small snzike around a
maze, trying to catch three enemy snzikes from behind without letting
them catch you. BnMerbund also sells the popular game Choplifter!,
in which you fly a helicopter behind enemy lines trying to rescue
64 hostages.

You should remember that you can buy more than just video
games. Chess is one of the most interesting games you can play
against the computer. Atziri's Computer Chess cartridge plays quite a
respectable game, and is easy to use.

Mzmy people enjoy adventure games. Unlike a video game, an zidven-
ture usually requires no fast shooting or quick action. Instead, it
places you in an imaginary situation, and asks you to find a treasure
or solve a problem. Two of the most popular are Zork, by Infocom,
and the Scott- Adams Adventure Series, by Adventure International.

Figure 2.2: The Serpentine game, by Br<fderbund Software.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You play an adventure by typing short commands, such as "climb
stairs" or "look under the table." If it can understand you, the com-
puter will do as you direct and then describe the results. You must
often be very resourcefiil to find your way around an obstacle in the
imaginary universe.

A final title worth mentioning is Shamus, by Synapse Software.
This game, shown in Figure 2.3, combines the best elements of video
and adventure games. You must explore an unknown dungeon, in
search of the keys and keyholes that will let you escape. In each room,
you must combat numerous monsters, shooting and dodging as you
wovild in a video game. You get die fast action of the video arcade and
the careful strategy of a dungeon adventure in a single game.

SERIOUS PROGRAMS

Atari and other companies sell many usefiil programis that can help
you in serious work as well. Education, home finance, and word
processing aire only a few of the things for which you can use your
computer.

figure 2.3: The Shamus game, by Synapse Software.

COMMERCIAL SOFTWARE 23

Many people buy programs to help their children's education. A
computer program can give a child individual attention and colorful,
animated illustrations of concepts. It can check his work each step of
the way, correcting his mistakes as he makes them. Some people also
feel that a child who is exposed to a computer at an early age will
be more comfortable with a machine that may play a large part in
his future.

Of the educational programs available for the Atari computers, my
favorites are those made by Spinnaker Software. That company's
Facemaker, designed for preschoolers, is one of the most entertaining
programs avziilable, yet it is eflfective in teaching basic skills of mem-
ory, recognition, and creativity. With this progrzim, your child can
put a face together on the screen, choosing from a selection of hair-
styles, eyebrows, noses, and mouths. He can also play a memory
game that challenges him to recall a lengthening series of facial
expressions. Spinnaker sells several other fine educational programs,
including Kindercomp and Snooper Troops.

If you want to introduce your children to programming on the
computer, you might consider Atari's PILOT. This is a simplified
programming language that lets a child explore the computer by cre-
ating graphic designs on the screen. It is sold as a plug-in cartridge.

Complex financial cadculations haunt many homeowners. A num-
ber of computer programs are available that can simplify such tasks as
calculating loan payments, making a budget, or keeping household
records. You may be amazed at how much easier it becomes to deal
with your financial problems.

Unfortunately, two of Atairi's best financial programs require a disk
drive. The Home Filing Mainager lets you keep records on electronic
index cards. When you need a piece of information, you czin search
through your file in any way you wish. For instance, you can retrieve
all of the entries that contain a certmn name or telephone number.
The program's attractive screen display even looks like a deck of
index cards, right down to the paper clip with which you mzirk impor-
tant entries. The Home Filing Manager would be useful for Einy type
of organized record-keeping, such as a mailing list or a teacher's
grade book. It sells for $49.95.

VisiCalc, which runs only on a disk drive and requires 32K of
memory, is another resource for financial management. This famous
program was the first of the electronic spreadsheets, which are popular in

24 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the business world. The program gives you a large, freeform table,
with many rows and columns. You type numbers and labels into the
ceUs of this table in any way you wish. You can then use the resources
of your computer to add figures or perform complex calculations.
This program would be useful for anyone who needs to keep track of
charts of numbers. Its suggested retail price is $199.95.

The other program that might help you is die AtariWriter word
processor. When you plug this cartridge in, your Atari computer acts
as an intelligent typewriter. You start by typing your text into the
computer's memory. You can then edit it by inserting words, deleting
sentences, and moving blocks of text around. Once your text looks the
way you want it, you type a command to print it. This program
requires a printer and either a program recorder or disk drive.

You can buy programs to do many other things with your Atari
computer. Music programs, such as the Atari Composer, let you play
melodies through your computer's sound system. Atari's Paint pro-
gram lets you draw colorful pictures on the screen.

HOW TO USE SOFTWARE

There's no real trick to using preprogrammed software. You just
turn the computer on, give some simple commands, and let the pro-
gram run. When the program needs some information from you, it
will usually ask you for it with a simple message on the screen. The
best programs are organized so that they actually help you organize
your thinking, and give you help when you need it.

With a cartridge, you just plug the program into the computer and
turn it on. On the Atari 600XL and 800XL, the cartridge plugs into
the large hole in the top of the computer, which is covered by a pair of
silvered doors. On die 1200XL and 1400XL, the cartridge slides into
the large hole along the left side. On die discontinued Atari 400 and
800 models, you will need to open the small door on the top of the
machine, then push the cartridge into one of die slots inside.

With cassettes or diskettes, you must load the program into the
computer's memory before you can use it. Start by connecting the
program recorder or disk drive to the computer, plugging the heavy
cable into the PERIPHERAL connection in back. Widi a cassette,
you must normally turn on the computer and type a command on
die keyboard to teU it to LOAD the program. With a diskette, the

COMMERCIAL SOFTWARE 25

computer will usually load the program automatically if you turn
it on with the disk in the disk drive. Most software packages include
detailed instructions on how to load the program. If you can't figure
out how to do this, refer to chapters 9 and 10 of this book.

If you aire using the 600XL or one of the other XL computers, you
may have trouble with a few prograims that were originally written for
the older Atziri computers (the 400 and 800). While most programs
are compatible with both the old and new models, a few are not.
In the cases where the programs are not compatible, the software
companies have generally prepared an updated version that will
work on all machines. You should contact your dealer to exchange
the program.

When using some older diskette programs on an XL model, you
must hold down the OPTION key as you turn on the computer. You
must do this because many older programs assumed that you had
removed the BASIC programming lamguage cartridge from the com-
puter before you loaded the program. Since BASIC is built into the
new computers and carmot be removed, you must use the OPTION
key to disable the language.

Once you have the program loaded, the going is easy. The better
progrzmis give you full directions right on the screen. You can usuaJly
read the instruction manu£il for further explzmation.

A LOOK AHEAD

This chapter has shown you how to use programs written and sold
by other people. In it, you have seen some of the things you can do
with your Atari computer with no experience whatsoever. By using
preprogrammed software, you can learn about your machine and
stEirt it working for you immediately.

You can do a lot more with your computer if you learn to give
commands and write programs on your own. You will be in direct
control of your machine, rather than foUowing along the lines some-
one else has laid out.

The next two chapters will show you some of the basics of using your
computer. You will learn how to use the keyboard and how to display
words on the screen. You will learn the fundamentals of your Atari
computer's graphics system, so that you can draw pictures as well.

Then, in Section Two, you will find out about real programming.

26 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

In this, you will combine the concepts you have learned in Chapters 3
and 4 into larger means of control. If you work slowly dirough all of
the examples, you will finish this guide with a good, firm knowledge
of your computer.

B CHAPTER THREE

The Keyboard
and the Screen

In this chapter, you will learn about your computer's keyboard. For
the most part, you will use it to type words and numbers just as you
would use a typewriter. However, there are several special keys that
you must also master so that you can control what the computer is
doing. These are aU described in the following pages.

At the same time, you will also learn how to control the words your
computer displays on your television screen. You will find that the
screen has two functions. It works with the keyboau-d, repeating every-
thing you type so that you can see what you have written, and it
displays the computer's responses to your commainds.

This chapter will introduce a simple command called PRINT,
which lets you display a single line of text or numbers on the screen.
As you learn more about this conunand, you will be able to position
the text where you want it and to clear unwanted clutter from
your screen.

Some of this may seem trivial. Why should you want to go through
all this trouble just to have the computer say HELLO? Why should
you care about conmiands that let you dear the screen? Be patient.
These are building blocks for the rest of the book. Before you can do
anything more complicated, you will need to know how to type com-
mands into your computer and how to interpret its responses. Once
you've passed this step, you'll be able to do many interesting things.

THE KEYBOARD

The Atari computer's keybojird is shown in Figure 3.1. It looks
very much like a normal typewriter, with a few things added. The

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

letters are aU in the same places, and only a few symbols have been
moved. You might notice a few unfamiliar keys at the left and right
sides of the keyboard, but for the most part, you can use it like a
typewriter.

Turn on your computer and your television set. If you are set up
correctly, your television screen will be dark blue, with the word
READY in the upper-left comer. If you can't get your screen to look
like this, reread the instructions in Chapter 1 .

The small white box under the R in READY is caUed the cursor.
Whenever die computer accepts letters from the keyboard, it uses this
square to mark the spot where your next letter will be displayed. Type
a letter— anything you want. It will appear where the cursor is, and
the cursor will move to the right.

There are a few peculiarities of this keyboard that are worth noting.
First of all, there are keys for die numbers and 1 . If you are used to
a typewriter, you may be accustomed to using die letters O and L for
these digits. You can't do this on the computer: you must give it
numbers when it expects numbers.

Auto-repaa is another new feature. If you hold down any key more
than a second, it will start to repeat. You could, if you wanted, type a
string of Zs across the screen by merely holding down the Z key The
repeating will stop when you release die key The auto-repeat feature
is particularly usefiil widi the space bar, at times when you want to
move to the middle of a line.

If you make a mistake as you are typing, you can correct it with the

Figure 3.1: The Atari 600XL keyboard

THE KEYBOARD AND THE SCREEN 29

BACK SPACE key, at the upper-right comer of the keyboard. When
you press this key, the cursor backs up one space to the left and
deletes the letter in that space. You can then type the correct letter
and continue as if nothing had happened.

As you type, you will notice that all of your letters are capitalized.
The AtJiri computers can display lowerczise letters, but in most cases,
they require that your commands be c^italized. Because of this, the
computer starts by using all caps.

If you want to type with lowercase letters, press the CAPS key, at
the far right edge of the keyboard. The computer now reads the let-
ters you type as lowercase. As with a typewriter, you can capitadize
individual letters by pressing one of the SHIFT keys. To switch back
into uppercase, press the CAPS key agziin. (On the Atari 400 and
800, you have to press the SHIFT and CAPS keys together to return
to uppercase.) The SHIFT key does not "release" the CAPS lock, as
it would on a typewriter.

To type symbols foimd on the upper half of the nonletter keys, you
must use the SHIFT key, even when you have the CAPS lock on. These sym-
bols include the exclamation point (!) zmd the quotation mark (").

You can use another specied key to produce an interesting eflfect.
Atari computers have an inverse-video key, which chamges the appear-
ance of the letters you type on the screen. This key, which looks like a
square with a diagonal drawn through it (B), is located next to the
SHIFT key at the lower right of the keyboard on the Atari 600XL
and 800XL. On the 1200XL, and the projected 1400XL and
1450XLD models, the inverse-video key is next to the BREAK key
on the row of silver keys along the top. On the older Atari models 400
and 800, this key has the Atari logo ( J|^) on it, and is located next to
the SHIFT at the lower right.

Try pressing the inverse-video key once. Then type some letters.
They will appear on the screen as blue letters in a white box, rather
than the standard white on blue. As you type, the white boxes join to
form a solid white background. Once you have selected this inverse-
video mode, you can type as you would in the normal mode, and
switch between uppercase and lowercase in the usual way. To return
to typing white on blue, press the inverse- video key a second time.

Figure 3.2 shows the four diflFerent styles of text you czin display,
using combinations of the inverse-video key and the CAPS key. The
first line was typed just Jis the computer was turned on. All of the letters

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

are capitalized and are displayed in normal video. For the second line,
the CAPS key was pressed once. The isolated capital letters within the
line were produced with the SHIFT key. Before the third line, the
inverse- video and CAPS keys were pressed, to show how your text
would look if you turned the computer on and switched immediately to
inverse video. For the fourth line, the display was again put into lower-
case with the CAPS key, but was left in inverse video.

Figure 3.2 also shows an important aspect of the Atari computer's
keyboard and screen. The fourth example is longer than the 38 char-
acters that will fit on the line. Rather than go off the right edge of the
screen when a line is too long, the computer automatically drops
down to the next line and continues. This is a nice feature, since it
lets you type commands longer than a single line. At times, however,
this can be somewhat unsightly, since words might be cut in the
middle (like the word "low/er" in Figure 3.2).

If you're accustomed to an electric typewriter, you might be
tempted to press the RETURN key at the end of a line you have
typed. On the computer, however, the RETURN key has a special

Figure 3.2: Lower<ase and inverse-video text on the screen.

THE KEYBOARD AND THE SCREEN 31

function: it enters the line as a command to the computer. So far,
you haven't learned any commands, so you shouldn't use this key.
If you do, your computer will print the word ERROR to show its
disapproval.

For right now, use the BREAK key instead of die RETURN key
to end each line. This tells the computer to drop down to the begin-
ning of the next line on the screen, without responding to the line that
you have just typed. On most Atari computers, you will find the
BREAK key at the upper-right corner of the keyboard. On the
1200XL and 1400XL models, it lies at die right end of the line of
silver keys along the top of the keyboard.

CONTROLLING THE CURSOR

You have seen that die letters you type are displayed on your televi-
sion's screen. The letters appear irom left to right, and die cursor slides
to the right in fixint of them, showing where you are on the screen.

You aren't locked into this left-to-right motion, however You will
often want to use editing commands to control what happens on the
screen. With certain keys and key combinations, you can move
the cursor anywhere you choose on die screen, and even go back to
correct letters you have already typed.

As you remember, the cursor shows where the next letter you type
will be displayed. If you move the cursor to another place, you can
type your next letter there. If you then continue typing, die letters will
resume their left-to-right motion from the place you've moved to.

To move the cursor, you will need to use the CONTROL key,
located just above the SHIFT on the left side of the keyboard. The
CONTROL key has no meaning by itself, but is always used with
another key. To use one of these specizd commands, press and hold
the CONTROL key As long as you hold it down, die computer will
interpret the rest of the keyboard as commands, rather than as letters.
To give the command, press the specific key that you want to use.

(If you press CONTROL widi one of die letters of the alphabet,
the computer will display a special graphics character. These can be used
to create pictures on the screen, but 3xe rather difficult to use. This
book will not treat the graphics characters, but you are free to experi-
ment with them.)

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

The most importaint of these special commands are the four arrow
keys located to the right of the letters on the keyboard. These arrows
move the cursor around the screen. The left arrow, for example, will
move it back one space; the down arrow will move it one line stradght
down. When you move the cursor with the arrow keys, it passes over
letters you have typed without changing them; it merely highlights
them to show you where it is. You can move the cursor rapidly by
holding down the arrow key. If you try to move the cursor off the
edge of the screen, it will reappear on the other side.

The left arrow, located on the plus key, is the most useful of the
four. To use it, you must press the plus key, while holding the control
key down. Note that the plus key serves three different purposes.
Typed alone, it is a plus sign ( + ). Typed with the SHIFT key, it is a
backwards slash (\). Typed with the CONTROL key, as described
here, it tells the computer to move the cursor one space to the left.

The four arrow keys will enable you to move the cursor anywhere
on the screen. You could, for example, use the arrow keys to move to
the bottom of the screen emd type a message there, as in Figure 3.3.

Figure 3.3: The arrow keys let you tyfx on other parts of the screen

THE KEYBOARD AND THE SCREEN 33

To see the arrow keys in action, type the following line on
the screen:

THIS LINE CONTAINS A MESTAKE

Press CONTROL and the left arrow key six times so that the cursor
is over the E. Then release the control key and type an I. The I
replaces the E and corrects the misspelled word.

This sort of correction is very useful. Unless you are a perfect
typist, you will certainly make mistakes from time to time. You can, it
is true, retype the line from the begiiming, but in most cases you will
find it easier to move the cursor back and just correct the error.

What if you leave a letter out? Suppose you have typed this
instead:

THIS LINE CONTAINS A MSTAKE

If you simply back up and type the missing I, it will appear on the
screen, but will replace a letter that you wanted. To avoid this, use
the INSERT command, located on the > key at the upper right
of the keyboard. Move the cursor back with the left-arrow key, until it
is over the S. Then hold the CONTROL key down and press
INSERT. The S and the letters foUowing will move one space to the
right, opening up the space covered by the cursor. Now you can type
the I into that space, and the correction is complete. When you need
to insert more than one letter, press the INSERT key several times.

You have already used the DELETE/BACK SPACE key to erase
mistakes. This key, when used in combination with CONTROL or
SHIFT, has two other meanings as well, which allow you to delete
letters in other ways.

To remove extra letters from the middle of a line without leaving a
space, use DELETE/BACK SPACE with the CONTROL key This
combination deletes the letter covered by the cursor and closes up
the space in the line. To see how this works, type yet another mis-
spelled line:

THIS LINE CONTAINS A MISSTAKE

Now back the cursor up with CONTROL and the left-arrow key
untU it is over one of the S's in MISSTAKE. Hold down the CON-
TROL key and press DELETE. The extra S will disappear and the
"TAKE" will move over to fill in the space. Note the difierences

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

between this and the simple BACK SPACE command: BACK
SPACE would have moved the cursor back and deleted the letter to
the left, and would have left a space in the line.

If you use SHIFT rather dian CONTROL with the DELETE/
BACK SPACE key, you will delete die entire line you are on. This is
very useful, if you need to clear unnecessary lines off the screen. You
must be very careful, however, not to delete an entire line with
SHIFT-DELETE when you meant to use CONTROL-DELETE or
a simple BACK SPACE.

To the other side of the INSERT key is a key marked CLEAR,
which lets you erase the entire screen. To use this function, hold down
the CONTROL key and press CLEAR. All the letters on the screen
will be erased, and the cursor will return to the upper left. You can
use this command to clear the slate anytime the screen has become
too cluttered for your taste. With some of the examples in this book, I
will ask you to type CONTROL-CLEAR before you give a com-
mand. If you don't do this, you won't be able to make your screen
match the figures I describe.

Figure 3.4 reviews the six editing controls. Practice them as you
work through the pages that follow. You will soon find it easy to cor-
rect mistakes anywhere on the screen.

We have now covered most of the important keys on the Atari key-
board. We can use type text, move the cursor, and make corrections
on the screen, using the Atari editing keys. We have avoided one
key — RETURN — because it has a very special purpwse. Let's turn to
it now.

KEY

BACK SB\CE by itseff

CONTROL + An arrow key

CONTROL + INSERT
CONTROL +

DELETE/BACK SPACE
SHIFT + DELETE/BACK SPACE
CONTROL + CLEAR

FUNCTION

Erases your last character, and
lets you type it over.

Moves the cursor.

Adds a space in a line.

Removes a space from a line.
Erases the entire line.
Erases the entire screen.

Figure 3A: The keys for editing the screen

THE KEYBOARD AND THE SCREEN

RETURN AND THE ERROR MESSAGE

Type a line of text such as the first line of Figure 3.5. Then press
the RETURN key. You will get an ERROR message, as shown.

What is happening here? The RETURN key has a special func-
tion. It tells the computer to take the line you have typed and read
it as a command. If it can understand the command, the computer
will follow it. If it cannot understzmd, the computer assumes you have
done something wrong, zmd prints an ERROR message.

You might object that your line made perfect sense. I agree, but the
machine does not. Computers require that you do things exacdy the
way they want. Most English sentences do not fit within the com-
puter's limited experience. Later in this chapter, you will learn some
commands that do mean something to the machine.

Don't be put oflF by the word ERROR. It is merely the computer's
way of saying it couldn't understand what you did. In this case, for
instance, we typed something that made perfect sense to us, but not to
the computer. The ERROR was its fault, not ours.

READY

THI5 IS HOT a C0MM6MD
RROR- THIS 05 MOT

COMMAND

Figure 3.5: You get an ERROR if you type a line the computer can't understand
as a command.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

ERRORS are part of the natural course of mastering your com-
puter, and you shouldn't be ashamed if you get a few. Your computer
IS very literal, and not at all forgiving. It will understand what you're
saymg only if you put it in exactly the form it requires. If you miss by
even a comma, the computer will not understand you.

The computer will usually help you find your mistake by putting an
mverse-video box around the first letter it can't understand. Your mis-
take will usually be to the left of this mark. At times, the computer
will display an error code such as this:

ERROR- 3

If you get one of these codes, you can look it up in Appendix C of this
book. The code number 3 represents a value error, which means you
probably used an illegal number somewhere in your command.

As you type the lines in this book, make sure that you copy them
exactly as they are printed, right down to the last quotation mark. If
you get an ERROR on any line you type, check it letter by letter
agamst what's printed here: you might have misspelled a word or lost
a punctuation mark.

Also, when you are typing a command line, make sure your com-
puter is set for aU capital letters, and is in its normal white-on-blue
display mode. If your letters are showing on the screen as lowercase
or mverse video, the computer will not be able to understand them in
a command. There is only one exception to this rule, and it is
discussed below. If you're not sure whether your computer is in the
proper mode for commands, press the RESET key or turn the power
oflF and beick on.

YOUR FIRST COMMAND: PRINT

So far, you have just been typing words and letters at various places
on the screen, using your computer like a memo pad. This is a good
way to learn about the keyboard, but it doesn't reaUy help you control
your computer. To do that, you will need to use a command that the
computer can understand.

In this section, you will learn your first command: the PRINT
statement. This lets you display a string of letters, numbers, and even
the results of calculations on the screen. For right now, you wiU only
use It to have the computer retype words and phrases, but later, you

THE KEYBOARD AND THE SCREEN

will be able to use it in many diflferent ways. By trying these simple
experiments with PRINT, you will also learn what a command is,
and how it aflFects the computer.

To give a command, you type a line on the screen zind press the
RETURN key. The computer will then read the line you typed and
try to follow its instructions. If it can't mjike sense of the line, the
computer will give you an ERROR message.

Type the following line, just as it is:

PRINT "HELLO"

Then press the RETURN key. The computer should respond
HELLO, as shown in Figure 3.6. (If you get an ERROR message
when you press the RETURN key, you have probably made a typing
mistake. Check to make sure you spelled PRINT correctly and put
the quotation marks both before and after HELLO.)

You have typed your first command! The computer recognizes
PRINT £is a keyword with a special meaning. When the computer sees
this word, it knows to search through the rest of the line and print

Figure 3.6: How the computer should react to your first command.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the message enclosed in quotes. It finds the word HELLO, so that
is the message it displays. Note that the computer does not reproduce
the quotation marks: they are merely a sign that the word HELLO is
the message to be printed. After displaying the message, the computer
then adds the word READY, to show it has finished and can accept
another command.

The results may not be impressive, but you have made an im-
portant step with your first command. You have asked the computer
to do something, in a way that it could understand. It read your
command and displayed the results on the screen. With this step, you
have begun to control your computer.

You can try many variations on this simple PRINT statement. You
can put anything you want between the quotation marks and it will
be printed exactly as you typed it. You can also use lowercase and
inverse-video letters within the quotation marks (but nowhere else in
the commzind). Also, any spaces you include between the quotes will
be included in the message the computer prints. Figure 3.7 shows
some of the things you can do.

seoDY

PRINT "YOU CAN PRINT ANYTHING."
YOU CAN PRINT ANYTHING.

READY

PRINT 'YOU can

in 1 ower c ase

You can print in lower c

READY

PRINT "YOU CAN_ PRINT n

YOU CAN PRINT t

READY

PRINT "If you wishj you can even print

on two 1 ines . "
If you wish, you can even prmt on two

I ines .

READY
PRINT

READY

Figure 3.7; Some examples of the PRINT statement.

THE KEYBOARD AND THE SCREEN 39

As you retype your variations on the basic PRINT statement, you
can practice your editing skills. Use CONTROL-t to move the cursor
up to one of the PRINT statements on the screen. Change or add
something inside the quotation marks, then press RETURN. Your
edited statement will be run again, and the computer will print the
new text over the old one.

This illustrates a useful aspect of your computer's screen display.
Any line that is still on the screen is "live" and can be used again. AU
you need to do is move the cursor back to the line you want to reuse,
make your change, and press RETURN — the cursor may be any-
where on the line. The computer will run the line again exacdy as it
appears on the screen. Any changes you have made will be cairried
into eflfect.

This is especially useful if you get an ERROR on a long com-
mand. Instead of typing it aJl over again, move the cursor back up
and correct the mistake in the line. You can then press RETURN
2ind reuse the line.

The fourth PRINT statement in Figure 3.7 runs across more than
one line. That is fine, as long as you type the entire command as a
unit, without pressing BREAK or RETURN. If you haven't finished
your message when the cursor reaches the edge of the screen, just
keep typing: the computer automatically goes onto the next line
down. You can type commands as long as three full lines of text. The
computer will warn you with a beep when you approach the limit.

The last line of Figure 3.7 shows an important use of the PRINT
command. If you type PRINT, but do not give any message, the
computer will print a blank line on the screen (actually two, if you
count the one that automatically appears above the word READY).

It is possible to give more than one command at a time. Instead of
pressing RETURN at the end of the first command, type a colon ( : ).
Then type your second commzind. In this way, you can add as many
commands as will fit within the limit of three lines on the screen.
When you're done, press RETURN and all of the commands will be
carried out in sequence.

Figure 3.8 shows how you can use blank lines and multiple state-
ments together to improve the appearance of your screen display. The
command begins with three simple PRINT statements, with no mes-
sage, to put three blaink lines on the screen. The fourth PRINT in the
command has a message, which begins with twelve spaces so that it

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

will be centered on the screen. A fifth PRINT adds a second line,
starting five spaces in from the left. A final PRINT puts an extra
blank line on the screen between the message and the word READY
that will appear.

The PRINT statement is one of the most important commands
you will learn. No matter how deeply you explore your computer, you
will always need to display the results on the screen. With the PRINT
statement, you can do this in any way you wish.

THE ESC KEY

The way we have been doing things, the screen remaiins cluttered
with your PRINT statement when your message is displayed. Sup-
pose you want to have only your message left on the screen when
you're done. You might be tempted to type the CONTROL-CLEAR
function as part of your message, something like this:

PRINT "{CONTROL-CLEAR} HELLO"

Figure 3.8: Blank lines and multiple PRINT statements can make your screen
more readable.

THE KEYBOARD AND THE SCREEN 41

Unfortunately, this doesn't work. The computer wiD dear the screen as
soon as you press CONTROL-CLEAR, and you will never get a
chance to complete the command with the message HELLO.

What we want is a way to store the CONTROL-CLEAR function
as part of the printed message. If you could do this, you could have
the computer wait to clear the screen until you have finished typing
the command. Then, when you finally press the RETURN key, it
would use the stored CONTROL-CLEAR function just before
it actually displays the message.

To do this, you can use the ESC key, located in the upper-left comer
of the keyboard. ESC stands for "escape," zind is used to delay the
effect of the next control character you type in a PRINT statement.

Let's try using the ESC key to PRINT the word HELLO on a
blank screen. Type the word PRINT and an opening quotation
mark. Then press the ESC key once. It will look as if nothing has
happened, since the ESC key itself is not displayed. Now press
CONTROL-CLEAR. Instead of erasing the screen inunediately, the
computer will display a curved arrow in a white box. This symbol
shows that CONTROL-CLEAR has been stored as part of the line,
to be used when the message is displayed. Finish typing the message
with the word HELLO and the closing quote, so that your command
looks like Figure 3.9.

Now press the RETURN key. The computer will print the message
HELLO, but before it does, it will dear the screen. Figure 3.10 shows
the result. This greeting is much more readable than Figure 3.6, in
which the PRINT statement was left on the screen.

CONTROL-CLEAR has actually become a character that is part
of the message. When the computer PRINTs the stored chziracter, it
acts as if you had just typed it at the point where it appeeirs in the
message. Any words you included in the message after the
CONTROL-CLEAR will be printed after the screen is cleared,
not before.

Note that you have to type die ESC-CONTROL-CLEAR as part
of the message enclosed within the quotation marks. If you put the
ESC-CONTROL-CLEAR anywhere outside the quotes, you will get
an ERROR.

The ESC key can be used to delay the effect of any CONTROL
key in a PRINT command. By storing combinations of the four
arrow keys, for instance, you can have the computer position text

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

anywhere on the screen. Each time the computer encounters a stored
arrow key, it will move a space in that direction before it continues to
print the line.

Figure 3.11 shows an example that stores the down arrow in this
way. Type the PRINT statement exacdy as it is shown at the top of
the screen. Wherever you see the ir symbol, type ESC and then
CONTROL4.

When you press the RETURN key at the end of the line, the com-
puter will print the message within the quotation marks, exactly as
you stored it. The first thing it sees is the stored down arrow. This
causes it to drop down a line before it displays the word UPPER.
After that, it encounters two more down arrows. These cause the
computer to drop two lines further down before it prints the word
LOWER. You could have achieved the same effect by using four sep-
arate PRINT statements, but it would have been much more compli-
cated. These ESC and CONTROL characters let you position your
messages much more easily.

Figure 3.9; Use the ESC key to store a control character in a line you're printing.

THE KEYBOARD AND THE SCREEN

THE SPECIAL FUNCTION KEYS

You might be wondering about the special keys such as START,
OPTION, SELECT, and HELP. On the Atari 600XL and 800XL,
these are square silver buttons to the right of the keyboard. On the
1200XL, 1400XL, and 1450XLD, these keys are spread across the
silver strip just above the keyboard. On the older Atziri computers,
the special keys are large yellow buttons along the right side of the
keyboard.

Unless you become an advanced programmer, you will use these
keys only when you are running programs you buy in the store.
Many of Atari's games use the START, OPTION, and SELECT
keys as one-button commands to replay, change the level of diflSculty,
or change the style of play. You will also use the OPTION key if you
need to disable the BASIC language as you load certain diskette pro-
grams. While it is possible to incorporate these keys into programs
you write, the procedure is too technical for this book.

HELP is a new key that Atari added when it redesigned its line of

Figure 3.10: ESC-CONTROL-CLEAR gives you a clean screen before you PRINT.

44 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

computers. At the moment, few programs are available that use this
key, but some are being plainned.

The Atari 1200XL and 1400XL computers also have four program-
mable Junction keys, labeled Fl through F4. These can be programmed
in the same way as the START, OPTION, and SELECT keys, but
the subject is, once agziin, rather technical.

The RESET key is the last of this group. Chapter 1 mentioned this
key as a kind of panic button that lets you regain control of your com-
puter when you don't know what it is doing. When you press the
RESET key, the computer stops whatever it was doing and switches
itself back to the state it was in when you first turned on the power.

As a "start-over" function, the RESET key is very useful. In the
following chapters, we will often do things that alter the screen display
or put the computer into strange modes. Rather than worry about
giving commands to make the computer return to its accustomed
manners, it is often easier to press the RESET key. The screen will
revert to its familiar blue, and READY will appecir in the comer.

Figure 3.11: By storing down arrows in a PRINT statement, you can display text
on more than one line.

THE KEYBOARD AND THE SCREEN

RESET, however, is a rather drastic step. It stops everything and
cancels many of the things you have done. That may be what you
wanted, but you should be sure before you press it. Be especially care-
ful when you have a progrzim stored in the computer's memory: the
RESET key can erase parts of long prograims.

OPTIONAL EXERCISES

Now that you have finished this introduction to your computer's
keyboard and screen, you might want to try your own hand at some
exercises. These aire optional, food for thought and avenues for further
exploration. I hope you'll try them, but you won't need to have done
them to read further in this book. Answers arc provided after the
Appendices in the back of this book.

1 . Use the arrow keys to type block letters made up of aster-
isks (*) on the screen. Move the cursor around and put
asterisks in the places where you wemt them to go. Remem-
ber to use only the arrow keys as you do this: you will spoil
your picture with an ERROR if you press RETURN.

2. On page 39, we saw a way to put more than one PRINT
statement in a single command, sepzirated by colons (:).
Use three PRINT statements to produce the following
message:

ONE
TWO
THREE

Can you achieve a similar result in a single PRINT state-
ment, using ESC and the arrow keys?

3. Replace the down arrows in Figure 3.11 with up arrows.
What happens when you press RETURN? Why does the
computer print the word READY right on top of the
statement?

SUMMARY

The keyboard and the screen work together. With the keyboard,
you can type letters and numbers that appear direcdy on the screen.

46 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You can also type CONTROL characters that move the cursor or
change the screen. You can use these CONTROL keys to edit the
hnes you have typed, to insert and delete characters, or to clear
the screen.

With the PRINT statement, you can ask die computer to display
messages on the screen. You can even store control characters in the
message, using the ESC key, so that die computer will clear die screen
before printing your message, or move the message elsewhere on
the screen.

You have covered a lot of ground in diis chapter You have learned
everydiing you need to know to use your computer's keyboard, and
you know how the computer controls the screen. Most important, you
have given your first command, and the computer gave its response.
In the pages ahead, we will cover many other commands that will
increase your control of your computer.

B CHAPTER FOUR

Atari
Graphics

If you have played Pac-Man or any other video game, you know
that your computer can display more than just letters and numbers on
your television screen. It can draw very colorful pictures, with impres-
sive animation eflfects.

In this chapter, you will learn about your computer's graphic dis-
play system. You will learn how to use coordinates, and how to plot
points on the screen. With a few simple commzinds, you will be able
to display letters, choose colors, and draw pictures.

The Atari computer has 16 diflferent modes for displaying text and
graphics. This chapter gives an overview of all of them, then covers
two in detail. The other modes will be discussed in Chapter 8.

Graphics are one of the most appealing features of your Atari com-
puter. I hope you will enjoy them.

WHAT ARE GRAPHICS?

The word graphics refers to any type of display you can put on the
screen other than plain text. This could mean a picture, such as a box
or a cube. It covdd be a har or line graph, such as those used to show
the state of the economy. Or it could be an animated game, such as
Pac-Man.

Of the computer's 16 graphics modes, 5 are called text modes,
because they are used to type letters rather than draw lines. You have
already seen one kind of text, the letters you have been typing on the
screen. The other four text modes shape their letters in different ways
and let you control the colors of individual letters. Using these other

48 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

text modes is somewhat complicated, however, so I will not describe
them until Chapter 8.

The other eleven are true graphics modes. All of them work on the
same principle: you can plot points individually or draw lines in any
direction across the screen. The modes differ in the number of colors,
and in the degree of detail, or resolution, available to you.

Why all these modes? Atari has realized that you will want to have
different features for different tasks. For a bar graph, you will want to
have thick lines, with relatively litde detail. For an intricate picture,
you will want to draw with fine lines, and show lots of detail. By
choosing different graphics modes, you can get just the amount of
detail you want.

Another rezison for having different modes involves the computer's
memory. When you ask the computer to use very high resolution and
many colors, it will need a lot of its memory to store the picture. If
you have one of the more expensive Atari computers or a memory
expansion, this is no great problem: even at the highest resolution, the
picture will only use a fraction of the memory. If you have only the
basic Atari 600XL, however, high-resolution graphics will use up
more than half of the available space. This might cause problems if
you are writing a complicated program. (With the programs in this
book, however, you don't need to worry about running out of mem-
ory space. AU of them will work on an unexpanded 600XL.)

So, your Atari computer lets you choose exacdy the combination of
features you need. If you need four colors and high resolution, you
can choose that. If you don't, you can save memory space by choos-
ing one of the modes with less detail or fewer colors.

For simplicity, this chapter will cover only two of the sixteen modes
available. The first is the normal text mode, which you have ah^ady
been using. The other is mode 1 , jmtr-color graphics, which lets you plot
detailed pictures on the screen.

These are the most important of the sixteen modes, and they will
serve weU for everything in this book. Chapter 8 discusses the other
modes, and gives some examples of what you can do with them.

TEXT: GRAPHICS

This is the mode you've been using so far. When you turn the com-
puter on, it is automatically set up for this kind of text, so you don't

ATARI GRAPHICS 49

need to do zinything special. At other times, however, you may want
to switch back into normal text from Einother graphics mode, so you
should know how to do it.

You switch graphics modes by typing the word GRAPHICS, fol-
lowed by the nvimber of the mode. The normal text mode is number
0, so you would type this:

GRAPHICS

Like the PRINT statement, this is a command, so you'll need to
press RETURN after you've finished. If you do this now, the screen
will just flash and say READY again: you're just switching from
mode to mode 0. Later on, you will type this when you want to get
out of the other graphics modes juid return to plain text.
Let's start by reviewing what we know about the text mode:

• With the PRINT statement, we can ask the computer to
display any message we put within quotation marks.
Unless we tell it to do something diflFerent, the computer
will display the message on the next line after the PRINT
command on the screen.

• By typing ESC-CONTROL-CLEAR at the beginning of
the text within the quotation marks of the PRINT state-
ment, we can have the computer clear the screen before it
displays the message.

• By typing ESC, CONTROL, and an arrow key within
the PRINT statement message, we can ask the computer
to move to another part of the screen before it types the
message. It is also possible to space downward by
PRINTing blank lines, but this is rather cumbersome.

THE POSITION STATEMENT

With the commands you have already learned, you can display a
message anywhere you choose on the screen. You have probably
found it rather difficult to do this, though, because you had to use
repeated ESC and CONTROL key combinations to move around
the screen. For this reason. Atari has given you another command:
the POSITION statement. This lets you move direcdy to any place
on the screen, before you stairt to PRINT your text.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

When you use the POSITION statement, you name the row and
column you want to move to. The computer thinks of the screen as a
grid 40 columns wide and 24 rows high, as shown in Figure 4. 1 . Each
row and column is given a number. The columns are numbered to
39, from left to right, and the rows Eire numbered to 23, fix)m top
to bottom.

With this numbering system, you can name any point in the grid
with a pair of numbers. With the first number, you choose the
column, to tell the computer how far to move in from the left edge of
the screen. With the second number, you pick the row, to say how fju-
you want to go down from the top of the screen. The point you
choose will be the box where that row and that column meet.

These two nvunbers are called the point's coordinates. The first num-
ber always names the column, or the distance over from the left. This
number is often called the x-coordinate. The second number always rep-
resents the row, or the distaince down from the top; it is cjilled the
y-coordinate. Progrzims in this book will often use the letters X and Y to
refer to horizontal and vertical locations. You can think of the paiir of
numbers as "X spaces over juid Y spaces down."

Figure 4.2 shows the coordinates of some points on the screen. The
square in the upper-left comer is always 0,0 — spaces over and

Column
1

Column 39

Row -» :::

Row 23 -► _.::::::::::

Fifmn 4.1: The Atari computer lays its screen out like a grid.

ATARI GRAPHICS

spaces down. This is true in the numbering system of every Atari
graphics mode. For the box at the upper-right comer, you need to
count all the way over to the 39th column from the left. Thus, the x-
coordinate of this point is 39. The y-coordinate remains 0, because
you didn't move any spaces down. We therefore write '39,0', mean-
ing "39 over and down." For the point at the lower-left comer, you
move spaces over and 23 spaces down, so its coordinates are 0,23;
To reach the point at the lower right, you move all the way over and
all the way down; its coordinates are therefore 39,23.

To name the point in the center of the screen, we need to do some
figuring. For the x-coordinate, we need to choose a number roughly
hjJfway between and 39. This could be either 19 or 20, but we'll
choose 19 so that we don't crowd the right-hand side of the screen.
Likewise, we need a value about halfway down the vertical scale of
to 23, so we'll choose 11. These coordinates (19,11) select the
box shown in the middle of Figure 4.2: it is 19 spaces over and 11
spaces down.

The POSITION statement lets you move to any coordinates on the
screen. If you follow this with a PRINT commauid, the message will
start at the point you have moved to.

If the coordinates happen to be on a line that already contains text.

0,0

39,0

0,23

39,23

Figure 4.2: Some sample coordinates in the GRAPHICS (text) mode.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the new message will be printed on top of what is already there. Since
that may be hard to read, it is often best to press CONTROL/
CLEAR before you give a POSITION command. If you don't do this,
you may not be able to see what the particular command has done,
and your television screen will not match the figures in this book.

Now that you understand coordinates, the POSITION statement
is very simple. To move to the point in the center of the screen,
simply type:

POSITION 19,11

If you press RETURN now, the computer will move the cursor to
these coordinates. But since you didn't give it anything to PRINT at
that point, it immediately moves on and displays the READY mes-
sage on the following line. The cursor then appears below that to
await your next entry. Now dear the screen and type:

POSITION 19,11: PRINT "CENTER"

Then press RETURN, and your screen should look like Figure 4.3.

The displayed word appears a little to the right of center on the
screen. This is because the POSITION statement sets the place only
for the beginning of the word. The C in CENTER appears in the box
(19,11); the word, however, continues on to the right. To place the
word exacdy in the center of the screen, you would need to start it
two or three spaces to the left. Try other coordinates in the POSI-
TION statement to center the word more exactly.

You will discover an important fact about the screen display if you
type the following line and press RETURN:

POSITION 20,0: PRINT "LEFT MARGIN"

The message Will appear at the bottom of the screen, two spaces to the
left of the normal left margin. Unless you specifically ask the com-
puter to POSITION a message in columns or 1, your typed lines
will always start in column 2. If you want to align your message with
the word READY, POSITION it in column 2.

In using the POSITION statement, you may occasionally get the
message

ERROR- 141

ATARI GRAPHICS

If you look up this error code in Appendix C, you will see that this
means "Cursor out of range." This means that you tried to move to a
point that was off the screen — one of your coordinates was too large.
Remember, the maximums for the X and Y coordinates are 39 and
23, respectively. When you get an ERROR, just CLEAR the screen
and start over

Experiment with the POSITION statement and the layout of the
coordinates. You will find it usefvd to think in terms of coordinates as
you start to use graphics. While the numbers of rows and columns
are different in the other graphics modes, the basic idea remains
the same.

FOUR-COLOR DRAWING: GRAPHICS 7

So far, we've only been working with letters. Now we're going to
do some real graphics, drawing pictures on the screen. In the remain-
der of this chapter, you will leam how to plot points, draw lines, and
make pictures on the screen.

Figure 4.3: Use the POSITION statement to display messages where you want
them on the screen.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

To draw pictures, we will be using graphics mode 7, one of the four-
color graphics modes. Don't worry about the modes that come between
and 7: we'll talk about them later.

To get into four-color graphics, type

GRAPHICS 7

and press RETURN. The top part of the screen wiU turn black and
the word READY will appear at the bottom.

The black part of the screen is devoted to the graphic designs you
are about to draw. You cannot move the cursor into this region, and
you cannot PRINT messages there, but you can use it to draw pic-
tures using points and lines. This region is called the grafMcs screen.

The graphics do not reach all the way to the bottom of the screen,
because the computer has to leave you some room to enter commands
and PRINT messages. These bottom four lines, where the words
remain, are called the text vuindow. In Chapter 8, you will learn a way to
extend the graphics down into this part of the screen as well. For now,
however, you'll need the text window to keep track of what you type.

There may be times as you use this book when you will want to see
more text than can fit in the text window. Especially in the program-
ming chapters, you may often want to look at an extended text that is
much too long for these four lines. When this happens, press RESET
or type a GRAPHICS command to return to the normal text mode.
Then you can display the extended text on a fiill screen once ^ain.

Your computer gives you four paintbrushes in this mode, one for each
color. Three of the four are drawing colors (orange, green, and blue),
while the fourth is an emsebrusk, which you will use to dear points fix)m
the screen. The erasebrush is actually a black paintbrush, drawing
with the background color: when you paint a point black, it disap-
pears. It is possible to change the brushes to colors other than orange,

(^m^wMt: W^twmM:

Figure 4.4: The colors of the four paintbrushes.

ATARI GRAPHICS

green, blue, and black, but for the moment these will suffice.

Before you start drawing, you must choose which of the paint-
brushes you wish to use. You do this by giving a COLOR command,
such as:

COLOR 1

This chooses paintbrush number 1, which is preset to orange. Figure
4.4 shows the colors of each of the paintbrushes.

Figure 4.5 shows the layout of the four-color graphics screen. The
top part of the screen is divided into a fine grid 160 dots wide and 80
dots high. You can refer to any dot on this grid by naming a paiir of
numbers as coordinates. The graphics coordinates follow the s£ime
system as the coordinates in the text mode, except that there aire more
rows and columns. In this mode, the x-coordinate runs from to 159
and the y-coordinate from to 79. Figure 4.5 shows the coordinates
you would use to plot the points in each comer of the graphics screen.

You can display either points or lines on the graphics screen, using
two commands. With PLOT, you can light up a single point on the
graphics screen. With DRAWTO, you can display a straight line
between two points.

0,0

n,79

Text Window

159,0

159,79

Figure 4.5: The layout of the four<olor graphics screen.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Generally, you will want to PLOT a point first. To do this, choose
the orange paintbrush, by typing

COLOR 1

Then type a line such as this:

PLOT 10,20

A pinpoint of light should appear in the upper-left corner of the
screen. If it didn't, check to make sure you have given the conunand
GRAPHICS 7, and that you have given a COLOR command to
choose a paintbrush. Also, make sure that your television is tuned
well enough that you can see this small point: it's finer than a period.

The '10,20' in the PLOT statement named the coordinates of the
point: 10 units over from die left, 20 units down from the top. These
are relatively small movements on the 160 x80-square grid of the
graphics coordinates; if you want to see how small, PLOT the point
0,0. Note that diis would have been quite a large distance in the nor-
mal text mode. Graphics grids tend to be finer than text grids.

You can PLOT as many points as you wish on the screen. Try a
few, to get a feel for the graphics coordinates. Each PLOT command
will result in another single point, provided you didn't exceed the
limits (159 for the x-coordinate, 79 for the y). If you do give a bad
number, you wiU get an error message, but you can continue typing.

With the PLOT command, you only get disconnected points. If
you want lines, you'll need to use the DRAWTO command. As its
name suggests, DRAWTO connects a line from the place where you
were, to the point you name as your destination. Try the follow-
ing commjind:

DRAWTO 159,79

An orange line will be drawn from the last point you plotted to the
lower-right comer of the screen. Depending on the angle from die last
point you plotted, the line may not look exactly continuous. This
"staircase eflfect" is caused by the limited resolution of a television's
screen and of the particular graphics mode you are using. Also, the
line may not be particvilarly orange, depending on the angle at which
you drew it. Vertical lines on the Atari tend to look pale or purplish,
compared to the luller colors of horizontal lines.

PLOT and DRAWTO work hand in hand. To draw a line, you

ATARI GRAPHICS

will generzdly first use a PLOT command to display the starting point
of the line. Then, you'll use a DRAWTO commzmd to draw the line
to its ending point. If you start right oflF with a DRAWTO command,
without first plotting a starting point, the computer will stairt the line
from the coordinates last used.

As an example, let's draw a horizontal line across the screen. Type
the following two commands:

PLOT 0,20
DRAWTO 159,20

After the first commzind, a point will appear a third of the way down
the left edge of the screen. With the second commzind, your computer
will draw a straight orjinge line across to the right side.

Suppose you want to draw a green line just below this. You will
need to change to paintbrush 2, then draw the other line. The follow-
ing three commands will do this:

COLOR 2
PLOT 0,30
DRAWTO 159,30

Note that the Y (vertical) coordinate has been changed to move the line
ftirther down. How would you plot a blue line just below the green?

If you give another DRAWTO command, the computer will con-
tinue the new line from the endpoint of the last one. A series of
DRAWTO commands is like drawing a number of straight lines on a
paper without lifting your pencil. There is no need to give a new
PLOT command to start each new segment. If, on the other hand,
you want to lift your pzuntbrush and start a new line not connected to
the rest, you will need to PLOT the starting point of the new line.
Think of it this way: DRAWTO traces a line from your present posi-
tion to your next point, while PLOT moves the paintbrush without
tracing a line.

To see these concepts in action, let's draw the box shown in Figure
4.6. Start by clearing the screen and choosing paintbrush 1, with
the lines:

GRAPHICS 7
COLOR 1

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Then give the foUowing five commands:

PLOT 10,10
DRAWTO 70,10
DRAWTO 70,70
DRAWTO 10,70
DRAWTO 10,10

With the PLOT command, a small point will appear near the upper-
left comer of the screen. Each DRAWTO commzind adds an edge of
the box, until the final one brings the pziintbrush back to the stairting
point. Since the four sides of the box are cormected, you didn't need
to lift the brush with a PLOT command.

There are several ways you can erase portions of your screen. The
simplest is to use the erasebrush, paiintbrush 0, by typing

COLOR

When you do this, your PLOT and DRAWTO commands become

Figure 4.6: Drawing a box on the screen.

ATARI GRAPHICS 59

erase commjinds, until you choose another padntbrush. Try typing the
following lines:

COLOR
PLOT 10,10
DRAWTO 70,10

The top of the box should disappear. (If it doesn't, you probably mis-
typed one of the coordinates. Try retyping the last two examples.)

If you want to erase the whole screen, however, the best thing to do
is to type

GRAPHICS 7

This resets the screen and lets you stewt all over. You may need to
type COLOR 1 again before you can stzirt paiinting.

That brings us to the end of our introduction to Atari graphics.
There is much more that you can do with your computer's graphics;
we have only scratched the surface. Chapter 8 of this book describes
briefly how you can use the 14 other graphics modes to produce
varied efiects. It will describe modes which allow you to use as mziny
as 16 colors on the screen at once. It will explain high-resolution graphics,
a one-color mode that lets you plot finer points than you cam with
four-color graphics. Finally, it will cover the special-text modes, which
give you other ways to print text, including color. But this is enough
for now.

OPTIONAL EXERCISES

1 . Using three POSITION and three PRINT statements in
one long command, display your name in three places on
the screen. Try to picture where the words will appear
before you press RETURN.

2. Rerun the commands you used to draw the orange box in
Figure 4.6. Then give a second series of commands, to
draw a blue box inside the first one.

SUMMARY

In the first three chapters of this book, you have foimd out how to
set up your computer and how to use it with commercial software

60 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

packages. You then learned to give the computer a command of your
own, which made it PRINT a message on the screen.

In this chapter, you have learned the essentials of your computer's
graphics system. You used coordinates in the POSITION statement to
place text mess^es where you wanted them on the screen. Then, you
learned to display points and lines in the four-color graphics mode. All
of these commands will be usefiol to you in die pages ahead.

The main thing you lack now is a way to save your commands.
The procedure for drawing a box, for instance, took seven separate
commands, each of which had to be entered from the keyboard. Sup-
pose you wanted to draw the box again. You would have to go
through those seven steps all over again.

There is an easier way, and that is die subject of Section Two.

Section

Programming

B CHAPTER FIVE —

Writing
a Program

In Section One of this book, you have seen some of the things you
can do with your computer. You have lezimed how to give commands
that print messages and draw simple pictures. As you gave your com-
mands, the computer immediately put them into action.

There has been one great limitation, however. You have had to
type every command in from the keyboard. If you made a small mis-
take in a series of commands, you have had to clear the screen and
start over fix>m the beginning. Also, if you wanted to repeat a certain
task (such as drawing a box), you have had to retype the whole series
of commands every time. Because of this, you have been limited to
very short procedures.

With a program, you can store a series of commzuids in the com-
puter's memory, then run them all as a group. When so instructed,
the computer reacts to the commzinds as if you had just typed them in
from the keyboEird. And when the computer has finished, the com-
mands are still stored, ready to be reused. If one of the steps was
wrong, or if you want to try a variation, you Eire free to alter the pro-
gram! and run it again.

Because programs let you design a procedure and reuse it many
times, they make it practical to do more complicated tasks on your
computer. If you were restricted to typing your commands one by
one, you would never want to try anything much more complex than

PRINT "HELLO"

With stored progrzims, you can easily combine many commands into
powerful procedures that you can use over and over. Many people
create programs involving hundreds of stored commands.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

As you wiU discover in future chapters, programs also give you new
kinds of control over what your computer is doing. In a program, you
can store words and numbers as you go along, and perform calcula-
tions. You can take a group of commands and run them over and over
in a loop, or have your computer do something only if a certain condi-
tion is fiilfiUed. With diese tools, you will be able to direct your com-
puter to perform complex tasks, then sit back while it does the work.

This chapter will show you how to store a program in the com-
puter's memory and how to run it. It will give a few short examples
to get you stzirted.

In learning to write programs, you wiU be mastering a programming
language. Computer designers long ago realized diat their machines
were too literal to understand complex English sentences, which are
often ftill of ambiguity. Instead, they decided to make up a system of
commands which are easy to learn yet specific enough for the com-
puter to understand.

With your Atari computer, you will be using Atari's version of a
language caUed BASIC. This stands for "Beginners' All-purpose
Symbolic Instruction Code"— if you can believe that. As its name
implies, BASIC is easy to learn and simple to use; because of this, it
has become die most popular language for smaU computers such' as
the Atari 600XL and 800XL. You have already been using some
commands from the Atari BASIC language (including PRINT
GRAPHICS, and COLOR). You will now be able to use these and
other commands in the programs that you write.
Let's see how it's done.

STORING YOUR PROGRAM

Your program consists of a series of commands, stored in the com-
puter's memory. When you are ready to use die stored commands,
you tell the computer to RUN the program. It then carries them out
one after the other

Before you do anything else, press RESET on your computer and
type the word

NEW

WRITING A PROGRAM 65

When you press the RETURN key, the computer will dear everything
from its memory and give you a dean slate to type your program.

You should give this NEW corrmiand every time you begin a new
program, even when you have RESET the machine. If you don't,
there may be program lines left in the computer's memory from some
other task. If you leave these lines in the memory, they may slip into
the program you are typing and disrupt it. You should make the
NEW command a habit.

You can store any Atari command as a line in a program, just by
placing a number in front of it. This is called a statement number, and it
serves as a signal that you wzmt the computer to store the conunand
rather than to put it into immediate action.

Back in Chapter 3, you typed your first command from the key-
board:

PRINT "HELLO"

When you pressed RETURN, the computer immediately did as it
was told, and displayed the word HELLO on the screen. Now let's
try a small variation. Type the following line:

10 PRINT "HELLO"

When you press RETURN, nothing seems to happen. The number
10 before the word PRINT told the computer just to store die Ime in
its memory. There is nothing special about the number 10; we could
have used any number.

You can check to see that the line is stored by typing another
conunand:

LIST

This tells the computer to display all of the lines it has in its memory.
In this case, there is only line number 10.

Now let's ask the computer to carry out the stored command. To
do this, type the word

RUN

and press RETURN. You get die same result as you did when you
typed the command widiout a number: the computer prints the word
HELLO on the screen. The action is delayed, however, until you give
the RUN command.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

w^r^ T ^ '^''""'^ *™^' *^ computer wiU repeat the word

tit^LLO. You can do this as many times as you like: your command
remams stored m the computer's memory until you turn the power
off or type NEW to dear the memory.

To add a second line to your program, just type another command
with a differem number in front of it. Again, this could be any num-
ber, but smce we used 10 as our statement number for our first com-
mand, let s use 20 for the second:

20 PRINT "HELLO AGAIN"

When you press RETURN, the second statemem will be stored in
the memory along with the first.

You can see how your program now looks by typing LIST. Your
computer will respond by typing all of the statements it has stored:

10 PRINT "HELLO"

20 PRINT "HELLO AGAIN"

Each statement is stored as a unit. The computer keeps them in die
order determmed by the statement numbers, regardless of the order
m which you typed them.

You now have two program lines in the computer's memory. What
will happen when you type RUN? The computer will search through
its memory and foUow the stored commands in sequence. After it has
finished, it will drop down and say READY once again. The result
will look like this:

HELLO
HELLO AGAIN

READY

You can add statements anywhere you want to in your program
Suppose we wanted a message between the two lines. AU we have to
do is type a command with a statement number between 10 and 20
and It wiU be filed in die proper place. We might, for example, choos^
the number 15:

15 PRINT "GOODBYE"

WRITING A PROGRAM 67

If you LIST the program, you can see that the new statement has
taken its place between the other two:

10 PRINT "HELLO"
15 PRINT "GOODBYE"
20 PRINT "HELLO AGAIN"

RUN the program. The new command will be carried out in the
order of its statement number:

HELLO
GOODBYE
HELLO AGAIN

READY

You can choose any number from to 32767 for your statement
numbers. You would do best, however, to use multiples often (10, 20,
30, etc.). This gives an ordered appearance to your program, and
leaves plenty of room for insertions.

To delete a line, just type the statement number followed by noth-
ing. Suppose you want to eliminate the last line, HELLO AGAIN.
Just type

and press RETURN. The statement numbered 20 will now be erased
from the computer's memory, as you can tell by LISTing the pro-
gram once again:

10 PRINT "HELLO"
15 PRINT "GOODBYE"

If you now RUN the program, the computer will use only the state-
ments that remain:

HELLO
GOODBYE

READY

If you want to change a line, type a new version using die same
statement number:

10 PRINT "HELLO THERE"

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

When you press RETURN, this line will replace the old version of
statement number 10. Your program will now look like this:

10 PRINT "HELLO THERE"
15 PRINT "GOODBYE"

It is important to remember that you can use each statement number
only once. If you duplicate a number, your new statement will replace
the previous one.

If you only want to make a minor change in a program line, you
can use the editmg functions you learned in Chapter 3. As you may
recall, you can change any command that you can see on the screen

PT.^T^ToS' "'°''' "^^ '"'^'' '° "' ""^^ y"""- ^0"-ection, and press
R^RN to enter the new version into die computer's memory.

,, J^°"./^" °"^y ^^" statements that are displayed on the screen
What if you want to change a line diat is not currendy displayed? You
could LIST the whole program again, but it is simpler to display just
the Ime you want to change. You can LIST a particular line with a
command like this:

LIST 15

Only line 15 will be displayed:

15 PRINT "GOODBYE"

Suppose you want to insert an exclamation after GOODBYE
Move the cursor until it is covering the second quotation mark as
shown m Figure 5.1. Now press CONTROL and the INSERT key
to open up a space in the line between die word GOODBYE and the
quotation mark. Then type an exclamation point, and you will have
the Ime the way you want it. (Review Chapter 3 if you are confiised
by these editing commands.)

You have corrected die line, but it is not yet stored in the com-
puter's memory. To store it, press the RETURN key: die corrected
Ime will replace die previous version and become part of die program
As far as die computer is concerned, you do die same diing by editing
a Ime as you would by typing it in from die beginning. The computer
doesn't care whedier you retyped die entire line or simply changed a
character m a line diat it had LISTed. No matter how die line got on
die screen, die computer takes it and stores it as part of die program

WRITING A PROGRAM

Use LIST to make sure your change has been incorporated:

10 PRINT "HELLO THERE"
15 PRINT "GOODBYE!"

Sometimes you will want to type a command such as RUN or
LIST when your cursor is not on a blank line. You can ask the com-
puter to give you a blank line by pressing the SHIFT and INSERT
keys together. You can also type CONTROL-CLEAR, if you don't
mind erasing the entire screen.

As a review, let's go back over the different ways you can use state-
ment numbers as you type your program.

• Add If you enter a line with a statement number that you
haven't used before, it will be stored as a new line of your
program.

• Insert If you type a line with a statement number between
two other numbers already stored, the line will be inserted
in the program at that point.

Figure 5.1: Editing a program //ne.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Replace If you enter a line with a statement number that has
already been used, your new version will replace the old.
Delete If you enter a statement number with no command
following, the computer will delete any statement stored

under that number.

You now know everything you'll need to type programs into the
computer You know how to add, insert, replace, and delete program
Imes. You know how to LIST your program, and how to run it.

It is time for a practical example.

THE BOX-DRAWING PROGRAM

At the end of Chapter 4, you typed a sequence of seven graphics
commands to draw a box on the screen. To review, try the same
series again:

GRAPHICS 7
COLOR 1
PLOT 10,10
DRAWTO 70,10
DRAWTO 70,70
DRAWTO 10,70
DRAWTO 10,10

The computer will shift into the four-color graphics mode and pro-
duce a picture diat resembles Figure 4.6 on page 58.

We're going to be writing a new program, so press RESET to
return to the text mode and type the command

NEW

to dear the computer's memory. Remember: a program line remains
m die memory until you either delete it, clean the slate with a NEW
conmiand, or turn the computer oflF.

What we're going to do now is simple. We're going to retype the
seven commands, but this time we'U give them numbers and store
them as a program. Type these lines:

10 GRAPHICS 7
20 COLOR 1
30 PLOT 10,10

WRITING A PROGRAM 71

40 DRAWTO 70,10

50 DRAWTO 70,70

60 DRAWTO 10,70

70 DRAWTO 10,10

When you're done, check your typing to make sure you haven't
made any mistakes. If necessziry, retype the line, or correct it with
your editing keys.

Now RUN the program. Almost instantly, your computer will shift
to the black graphics screen and draw an orange box. The result is the
same as Figure 4.6, but the computer did it all in one step, rather
than seven.

With a procedure as long as this, the advamtages of stored pro-
grzims become evident. You cam correct a mistake by editing, rather
than going back to the begiiming. You can draw the box ageiin with a
single RUN command, rather than retyping all seven steps. As we go
on, you will discover other advantages to writing programs, zmd other
uses to which they can be put.

HINTS ON WRITING PROGRAMS

As you write your own programs, you will need to develop some
habits that make your work easier. Cleair thinking and good organiza-
tion are often the difference between successftil programming and
computer nightmares. Here are three rules to follow:

1 . TTiink it through. You will save yourself a lot of agony if you spend
a few minutes planning out your program before you start typing it.
You don't have to write the program out in fiill unless you feel the
need, but you should sketch out the route before you embark on your
journey.

2. Be neat. There is nothing more frustrating than trying to make
corrections in a sloppy program. If you organize your progreim so
that it is easy to read and logical in its layout, you will understand it
much better when you go back to change it. Use a regulzir pattern in
your statement numbers, such as 10, 20, 30.

3. Leave notes to yourself. Your computer lets you leave comments in
the middle of your program. Comments that explain what you are
trying to do will make your program much easier to understand.

To leave a note at a certain point in your program, type an unused
statement number and the letters REM (for "remark"). You can then

72 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

type anything you choose on the rest of the line. For example, at the
beginning of the box-drawing program you might want to add a line
that aimounces its purpose. Try the following:

5 REM PROGRAM TO DRAW A BOX

When you LIST the program, this remark will appear at the head.
When you RUN the program, however, the computer will ignore
the remau-k.

You may feel these remarks are a waste of time, if you are only
writing programs for yourself: you know what you're doing, and the
computer doesn't need the explanations. Don't kid yourself. Even if
you understand the program perfecdy now, you won't remember it
six months from now if you decide to revise it. Also, if you give your
program to a friend, you will want to include enough information to
explain what you have done.

Figure 5.2 shows how remarks can be used to explain the box-
drawing program. Asterisks were placed around the tide of the pro-
gram to set it apart. Blank REM statements in lines 8, 24, and 34 are
used to separate the program into its major parts. The REM state-
ments 9, 25, and 35 name the parts. By judiciously using remarks in
this way, you can make the structure of your program much more
visible.

DEBUGGING

No matter how careful you are in writing your programs, you will
certainly have troubles from time to time. Your program doesn't work
exacdy die way you want it to, or you get an ERROR when you try
to RUN it. This is known as a bug in your program, and you are
faced with the task of debu^ng.

The first diing to check is die ERROR message, if diere was one.
If, for example, your screen looks like Figure 5.3 when you try to
RUN the box-drawing program, you have a pretty good idea where
to look. The computer was partway through the program, then some-
thing went wrong when it reached statement 60.

What does this ERROR message mean?

ERROR- 3 AT LINE 60

/v^c wiuiv,i ucictc 11, ucaii tne siaie wirn a iMh.w

WRITING A PROGRAM

The 3 in this message is a code that describes what the problem was.
To translate it into English, look the code up in Appendix C of this
book. You will find that 3 meains "Value Error." This is one of the
most common errors on the Atari computer, and it usually comes
from typing an incorrect number, or a number too large for the place
where it's being used. This is the ERROR you get if you try to
PLOT or DRAWTO a point with a coordinate that is too large for
the graphics screen.

Once you know where the error occurred, LIST the program and
check it over. In this case, line 60 is the most likely place for the error,
but you should look at the other statements as well. Often a mistake
in an earlier line will cause problems in a perfecdy good line later on.

Check the program over, line by line. Pay particular attention to
details; a missing comma or a misspelled word can block an entire
program.

Check to make sure that no lines are missing. It is easy to leave out
a commcind as you are typing, or to erase one by typing the wrong

READY 1

LIST

5 REM » PROGROM TO DRAM ft BOK »

8 HEM

9 REM --5ET UP FOUR-COLOR GRAPHICS —

10 GRAPHICS 7

28 COLOR 1

24 REM

25 HEM --MOUE TO UPPER-LEFT CORMER--

se PLOT xe^xe

34 REM

35 REM DRAM FOUR SIDES OF BOK

40 DRAHTO 70, XG

50 DRAMTO 78,70

60 DRAHTO X0, 78

70 DRAMTO XO^XO

READY

■

Figure 5.2; Use REM statements in your programs to make them more readable.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Statement number. Make sure also that no unwanted lines have
slipped into your program. For example, you might have forgotten to
give a NEW commjmd before you started typing.

If you still can't find your mistake, have a friend check your pro-
gram over. Often smother person will be able to spot a small mistake
that you had missed. Try explaining your program to someone else.
This will frequently uncover a gap in your reasoning.

After you have exhausted all the other ailternatives, you may be
tempted to think it is the computer's fault. This is unlikely. Com-
puters are very reliable in following their instructions, and they are
rarely at fault for the errors. Usually, it is a breakdown in the com-
munication between you and the machine: what you thought you
were writing is not what the computer took you to mean.

You should suspect the computer itself only if it starts behaving
erratically on a variety of programs, or on programs which you've
already tested out. If your machine is defective, the problems will usu-
ally be obvious, and will affect the computer's overall functioning.

Figure 5.3: A typical ERROR message.

WRITING A PROGRAM

rather than just a single program. Check with your dealer if you feel
that your machine has gone bad.

Have courage. It can be very difficult to debug a program, so don't
become frustrated. Often when you do find the bug, it will seem like a
silly mistake. But don't blame yourself for having overlooked the
error. It happens to everyone.

CASSETTE STORAGE

You can store only one program at a time in your computer's
memory. If you wish to use a diflferent program, you must erzise the
first one by typing NEW. Your program is also lost when you turn
the power oflF.

This is a problem if you have written a program that you want to
use over and over. Unless you can leave the computer on indefinitely
and use it only for that one program, you will need to retype the pro-
gram every time you want to run it.

If you have an Atari Program Recorder or Disk Drive, you can
save your programs permanentiy and load them back into the com-
puter's memory whenever you want to use them. This is very handy —
well worth the investment if you plan to write long programs.

At this point in the book, I wiU not take the time to tell you exactiy
how to use the program recorder or the disk drive. Instead, I have
collected all of the detailed instructions in Chapters 9 and 10. If you
want to learn about your cassette recorder or disk drive right now,
skip ahead to these chapters: you can read them without knowing the
material in Chapters 6 through 8.

Just for convenience, however, I will explain the simplest of these
operations: saving a program on cassette tape and loading it back in.
If you get hung up on these directions, or if you are using a disk drive
rather than a cassette recorder, you should read the detailed instruc-
tions in Chapters 9 and 10.

Start by connecting your Atari Program Recorder. You can use
either the new model 1010 or the old model 410; both will work with
any Atari computer. You must plug the recorder into a wall oudet,
then connect its special black cable to the large hole on the back of
your computer labeled PERIPHERAL. Refer to Chapter 9 if you
don't know how to do this.

You can now save the program in the computer's memory on a

76 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

regular cassette tape. Any tape will do, even the cheapest. Put the
tape in the recorder and rewind it to the beginning. Then type the fol-
lowing commzind:

CSAVE

This tells the computer that you want it to save the progrzim on the
cassette. When you press RETURN, the computer will beep twice
through your television as a cue for you to record. Press PLAY and
RECORD on the program recorder. Press RETURN again.

Your computer will start to whisde: this is the actual sound it is
recording on the tape. It meains nothing to you, but to the computer
it contains all of the information needed for your program. The re-
cording process takes about 20 seconds (more if your program is very
long). The recorder will stop automatically when it is finished, and
the computer will say READY. Widi your program on tape, you can
safely erase it from the computer's memory or turn the computer off.

To load the program back into the memory you must use a similar
procedure. Type

CLOAD

and press RETURN. The computer will beep once, asking you to
start the recorder. Rewind the tape and press the PLAY button. Press
RETURN again. The recorder will turn aind the computer will load
the program. When it has finished, the recorder will stop and the
computer will say READY. You can now LIST the program to make
sure it was loaded correctly, then you can RUN it again. Note that
the program you load will replace any program you had previously
stored in the computer's memory.

If you have any problems with these brief directions, please read
Chapter 9. The descriptions there are much more complete, and con-
tain other helpfiol suggestions. This is just to get you started.

OPTIONAL EXERCISES

1. In the box-drawing program on pages 70-71, add a state-
ment that will make the last line in the box blue. You will
have to choose a statement number between 60 and 70.

2. Add a line at the end of the same program, to draw a

WRITING A PROGRAM 77

diagonal line from the upper-left comer of the square to
the lower right.

The following program contains three mistaikes:

PRINT "HELLO THERE"

GRAPHICS 7

COLOR 1

PLOT 10,734

DRAWTO 0,0

DRAWTO 10

Find the bugs.

A 0\/

In this chapter, you have seen how you can string your conunands
together to make a progrzim. You can store as many commands as
you like in the computer's memory, and RUN them all as a group.
You can use the LIST command to review the progrjmi you have
stored, or to make duinges in it.

Programs have many uses, as you have zJready seen. Since they
can be changed and reused as often as you like, programs allow you
to try much more complicated operations tham you would if you
needed to type the commands each time. If you have a cassette
recorder, you can even save the program permanently for future use.

In the next two chapters of this book, you will learn techniques that
make this concept much more flexible. You will increaise your control
over your computer and discover its true power.

B CHAPTER SIX

Using
Variables

So far, you have been giving very specific instructions to your com-
puter. You have told it to PRINT a word or to PLOT a point at a
given coordinate. If you want to PRINT a different word or PLOT a
different point, however, you have to type a whole new command.

You can make your commands much more flexible if you are less
specific. Your computer allows you to replace numbers with variables,
labels which can stand for any number you choose. As you RUN the
program, you specify what numbers the variables will represent.
Later you can run the program with the variables stzinding for differ-
ent numbers.

Variables also let you use your computer for calculations. Your com-
puter can do arithmetic quickly and accurately, then display the results
on the screen. This can save you many hours of work. At the end of
this chapter, you will learn how you can make variables represent let-
ters as well as numbers, so that you can manipulate words as well.

In this chapter, we will try out severed variations on the program
that draws a box on the screen. The idea remains the same as the pro-
gram we developed in Chapter 5; this time, however, we will use vari-
ables to stand for the coordinates. This will make it much eaisier to
change the shape and location of the box the program draws.

STORING NUMBERS

Computers generally prefer to do their thinking with numbers,
rather than words. In most of our commands, we have used one or
more numbers to tell the computer what we wanted. To PLOT a
point, for instance, we used a pair of numbers to name its coordinates.

To use a fixed number such as 46, you just type its digits. In

80 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

general, you can type the number exactly as you would normally
write it. When it is greater than 1,000, however, you should leave out
the comma that separates the thousands from the hundreds.

These are aH fixed numbers. You can type a fixed number in any
command, but it is used immediately rather than stored. If you wjint
to reuse the number, you will have to type it out again.

There is a way, however, to store a number in your computer, so
that you can reuse it without retyping it. To do diis, you give the
number a name. You can call it anything you want, just as long as
you begin the name with a letter.

You might, for example, want to call it FRED. To store the num-
ber 7, all you need to do is type

FRED =7

When you press RETURN, your computer sets aside a place in its
memory for the variable FRED. It then stores the number 7 in that
place, so that you can refer to it by name.

You cannot turn this equation around, as you can in normal
arithmetic. If you type

7= FRED

you are saying, "Take the number FRED and store it under the
name 7." This makes no sense to the computer, and you will get an
ERROR.

The computer actually thinks of the equal sign as a command.
When it sees this sign, it looks for whatever value is to the ngA/ of the
sign, then stores that value in the name to the left. This is called an
assignment statement.

The name FRED now represents the number 7. You can see this
by asking the computer to PRINT the value:

PRINT FRED

If you do this, the computer will display the nvimber 7. (If it displays
or something else, you have probably mistyped something: try
typing the laist two commands again.)

You can now use the name FRED anywhere you would normally
use the number 7. You could switch into the four-color graphics mode
by typing

GRAPHICS FRED

USING VARIABLES 81

You could PLOT the point 7,7 by typing

PLOT FRED.FRED

You could even store the number 7 in another variable named JANE
by setting it equal to FRED:

JANE = FRED

Let's look a litde more carefiolly at the statement

PRINT FRED

When you pressed RETURN, the computer displayed the value you
had given to the name FRED. Notice that the result is very different
from what you would get if you said

PRINT "FRED"

If you do this, the computer will type the word FRED on the screen,
rather than its value. With the quotes, the computer reads "FRED"
as a message to be displayed. Without the quotes, it reads FRED as
the name for the stored number, and displays its value.

The stored number is czilled a variabk, because you can chzinge its
value. To do this, you merely store a new value. The computer will
accept it, and replace the old value with it.

To chzinge FRED's value to 19, type

FRED = 19

The old value (7) will be lost, and FRED will mezm 19 from now on.
You can check this by giving the command

PRINT FRED

FRED will continue to mean 19 until you store another value or turn
the computer off.

Until you use a variable, the computer assumes its value is 0. You
can see this by printing a name you haven't used:

PRINT IRVING

The computer will respond with the number 0, since you haven't
given a value to the variable IRVING.

So far, we have been using people's names, like FRED, JANE,
and IRVING. It is generally best, however, to use a word that

THE EASY GUIDE TO YOUR ATARI 600XL/eOOXL

explains what the variable represents. If you have a variable that
counts the number of times you have done an operation, you might
call it COUNTER. For graphics, you might want to caD the coordi-
nates of a point X and Y, so that you can simply say

PLOT X,Y

When you are plotting more than one point, you might give your
coordinates difierent names all starting with X and Y. For example, if
you were drawing a line from one point to another, you might call the
starting point XSTART, YSTART and the ending point XEND,
YEND.

There are only a few restrictions on the names you can choose for
your variables. The name can be as long as you wish, but it must
begin with a letter. The rest of the name can contain numbers, but no
special symbols. You may not use lowercase letters or inverse video in
your variable nsunes. Finally, you may not start your variable name
with an Atari command word, such as PRINT or PLOT The com-
puter would have no way to tell the difference between your variable
and the command.

Within these guidelines, you can use any name you like. The
names in the left column of Figure 6.1 show some of the possibilities.
The right column shows some examples of illegal words. You don't
need to be too concerned, though, since most of the names you would
use are legal.

In these experiments with stored values, you have seen how vari-
ables work. You can store a number in any variable you choose, then
use the variable's name instead of the number. If you want to change
the number, you merely store a new value. The variable will repre-
sent the new value until you change it again.

LEGAL

ILLEGAL

(REASON)

DATE

123

Doesn't begin with a letter

NUMBER

BILL'SBILLS

Contains a special symbol

SIDE4

Wilhelm

Not capital letters

DELAY

THE NUMBER

More than one word

GRAPHICS

Atari command word

Figure 6.); legal and illegal variable names.

USING VARIABLES 83

VARIABLES IN PROGRAMS

When you're just typing simple commeinds, variables don't seem
all that useful. As long as you are typing each command as you go,
you might as well use the fixed number instead.

In programs, however, variables become very important. With
them, you can write a general procedure that accomplishes a certain
task, without knowing what specific numbers you will be using on any
particular occasion. When you RUN the program, you can plug in
the specific numbers you want. If you later want to perform the same
task with different numbers, you simply RUN the program again
with the new values.

To see how this works, let's return to the program to draw a box
that we developed in Chapters 4 and 5. To start off, give a NEW
command and type the program back into the computer's memory, as
follows:

4 REM ******************

5 REM * PROGRAM TO DRAW A BOX *

6 REM ******************

8 REM

9 REM -SET UP FOUR-COLOR GRAPHICS -

10 GRAPHICS?
20 COLOR 1

100 REM

1 01 REM - MOVE TO UPPER-LEFT CORNER -
110 PLOT 10,10

120 REM

1 21 REM - DRAW FOUR SIDES OF BOX -
130 DRAWTO 70,10

140 DRAWTO 70,70
150 DRAWTO 10,70
160 DRAWTO 10,10

This program is exactly the same as the one shown in Figure 5.2,
except that the statement numbers have been changed in the second
half. This was done to make rOom, because we're going to add some
statements between numbers 20 Jind 100. If you want to save yourself
some typing, you can skip some of the REM statements, which do
not affect the operation of the program.

In every version of this progrzim that we have used, our box has

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

always been the same size, and it has always been in the same loca-
tion. The coordinates we used to set the comers are fixed numbers.
The box starts at the coordinates 10,10— a point near the upper-left
comer of the screen. Each edge is then drawn to a fixed point, until
the final edge retums to the starting point. Figure 6.2 shows the fixed
coordinates of each comer of the box.

We can make our program more general so diat it can draw a box
of any size, shape, or location. Instead of using fixed coordinates for
the comers, we can use variables.

Figure 6.3 shows the names we'll be using for each of the points.
These names were chosen to explain the numbers they will represent.
XLEFT, for example, gives the x-coordinate of the left side of the
box. Note that each of the variables is used in the names of two diflFer-
ent comers: XLEFT is the x-coordinate of both the upper-left and
lower-left comers.

Let's begin changing the program from numbers to variables.
Instead of giving another NEW command and retyping everything,
just type the new lines that you want to add. As you may recall, when
you type an additional line while you have a program stored in the
memory, the new line will be inserted into the program. If its state-
ment number matches one already in the program, the old statement

Figure 6.2: With fixed numbers, you can only have a square of one size.

USING VARIABLES

will be replaced. In this way we wiD be able to change certain parts of
the pit)gram without retyping the whole thing.

There are times in this chapter when you will RUN a program that
will leave the screen in the graphics mode. You can type commands in
the text window at the bottom of the screen, but you will usually find
it easier to return to the text mode. You can do this either by typing
GRAPHICS 0, or by pressing RESET.

Before we can use the four variables to name the comers of the
box, we need to store their values. We could choose any numbers we
want, but let's stzirt with the coordinates we've been using: 10 and 70.
Insert the following five lines into your program:

30 REM -STORE VALUES IN VARIABLES -

40 XLEFT=10

50 XRIGHT = 70

60 YTOP = 10

70 YBOTTOM = 70

Now that we have the vjilues stored, we can retype the PLOT and
DRAWTO statements using the variables instead of fixed numbers.
Type five lines as follows:

110 PLOT XLEFT.YTOP
130 DRAWTO XRIGHTYTOP

XLEFT,YTOP

XRIGHT,YTOP

XLEFT,YBOTTOM

XRIGHXYBOTTOM

F^n 6.3; We will g/ve these variable names to the comers of the box.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

140 DRAWTO XRIGHT.YBOTTOM
150 DRAWTO XLEFT.YBOTTOM
1 60 DRAWTO XLEFT, YTOP

These lines will replace the PLOT and DRAWTO statements in the
original program.

LIST the program to make sure that all of your new statements
have been stored correcdy. Check the lines you have just added, to
make sure they are accurate. The program should exacdy match the
listing shown in Figure 6.4.

When you're sure you have it right, RUN the program. You
should get exacdy the same results as you did with the original ver-
sion: an orange box on the left half of the screen. This is because the
variables used in the PLOT and DRAWTO statements contained
exacdy the same values (10 and 70) as the fixed coordinates did in the
original version.

With the variable version, however, you Ccin change the shape of

)ET UP FOUR-COLOR GRftPHIC5--

REM —-""--
,; REH * PROGRAM TO DRAW (
6 REM

8 REM

9 REM -iET UP FOUR-COLOR

10 GRAPHICS 7
20 COLOR i

3 REM S T O RE U (3 L U

4 H L E F T :- 1 O

5 X R I G H T r: 7 «
60 YT0P=10

70 YBOTTOM '0

100 REM

101 REM --MOUE TO UPPER-L
110 PLOT KLEFT.VTOP

120 REM

121 REM --DROW FOUR SIDES
130 DRflWTO HRIGMT,YTOP
140 DROWTO XRIGMT,YBOTTOM
150 DRAWTO HLEFT , YBOTTOM
160 DRAWTO KLEFT.YTOP

IN UARIABLES--

CORMER-

OF BOK--

Figure 6.4: The revised version of the tmx-drawing program.

USING VARIABLES

the box by modifying the statements that store the values. Replace
statements 40 through 70 with the following:

40 XLEFT = 20

50 XRIGHT=140

60 YTOP = 35

70 YBOTTOM=45

If you LIST the program, you will see that the new statements 40
through 70 have replaced the old. When you RUN the program,
these new values produce a thin horizontal rectangle across the
screen, as shown in Figure 6.5. By choosing other values for these
four variables, you cam produce rectangles of any dimensions. Type
and RUN these two other combinations:

40 XLEFT = 75

50 XRIGHT = 85

60 YTOP = 10

70 YBOTTOM = 70

40XLEFT =
50XRIGHT=159
60YTOP =
70 YBOTTOM = 79

Figure 6.5; Using variables, you can draw a tmx of any size.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

With the example at the left, you will get a thin vertical box. With the
values at the right, the computer will draw a yellow rectangle around
the entire graphics screen.

Try out some values of your own in this program. You can change
any of the variables you want, then RUN the program. The com-
puter will draw a box according to the dimensions you give. You can
choose any numbers you want for the variables, as long as you don't
exceed the limits on the X and Y dimensions of the graphics screen.
X must fall between and 159, and Y between and 79, or you will
get an ERROR.

Notice that the variable XLEFT occurs three times in the PLOT
and DRAWTO lines of the program, yet you can use one new assign-
ment statement to change its value in all three places. No matter how
many times a variable occurs in a program, one new assigimient will
change its value throughout.

By using variables, we have made a real improvement over the
earlier versions of this box-drawing program. We can now tell the
computer exacdy where to put the four edges of the box and how
large to make it. More important, we were able to design the pro-
gram to perform a general task, then change specific numbers without
rewriting the entire program.

THE READ AND DATA STATEMENTS

When you have wanted to change die value of a variable you are
using in a program, you have needed to go back and change the state-
ment where you stored it. This is all right if you rarely make changes,
or if diey only involve one variable. If, however, you frequendy need
to change many values in a program, there is a better way: your com-
puter lets you assign values to several variables in a single statement.
You do this with two new commands: READ and DATA.

READ and DATA always work as a pair, and you cannot use one
without die odier. In die READ statement, you give die computer a
list of variables to which you wish to assign values. You then type a
DATA statement widi a corresponding list of the numbers diat you
want to store. The computer takes the first value from the list in the
DATA statement and assigns it to the first variable in the READ
statement. If there are more variables to be filled, it returns to the
DATA statement and stores the second value in the second variable in

USING VARIABLES 89

the READ statement. This process continues until all of the vEiriables
have been filled. You must have at least enough values in the DATA
statement to fill aU of the variables. If you run out of numbers to read,
you will get an error.

To see how this works, let's revise lines 30 through 70 of the box-
writing program to use READ and DATA statements. Type the
follovmig lines:

30 REM -READ VALUES INTO VARIABLES -
40 READ XLEFT,XRIGHT,YTOP,YBOTTOM
50 DATA 10,70,10,70

When you're done typing this, add two more blank lines, to delete
unwanted statements left over from the previous version:

60
70

LIST the program: it should match Figure 6.6. When you RUN the
program, you will get the same results as before — a large yellow box
on the left half of the screen.

The four vairiables in the READ statement correspond exactly to
the four values in the DATA. When the computer comes to the
READ statement, it plugs the first nvimber from the DATA statement
(10) into the first variable (XLEFT). It then continues, with the other
variables, reading 70 into XRIGHT, 10 into YTOP, and 70 into
YBOTTOM. These two lines have the same effect as the four lines
we replaced:

40 XLEFT=10

50 XRIGHT =70

60 YTOP=10

70 YBOTTOM = 70

The READ and DATA statements, however, are easier to type and to
understand.

The great advantage of READ zind DATA statements is that you
can easily change to new vzdues. If you want to draw a thin horizontal
box, as in Figure 6.5, you need to type only a new DATA statement:

50 DATA 20,140,35,45

When the computer draws the box, the corners are set by the new
values.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You can experiment with many different numbers in this DATA
statement. In the previous version, you had to replace four lines, but
here you need to retype only the one DATA statement. Try these sets
of values:

and

50 DATA 75,85,10,70

50 DATA 0,159,0,79

You should get yellow rectzingles of other shapes, just as you did on
page 87. Try some other values of your own.

INPUT: GETTING VALUES

FROM THE KEYBOARD

There is a third way to assign numbers to Vciriables, which is even
more flexible than the READ statement. This is the INPUT state-
ment, which has the computer pause in the middle of the progrzim

PROGROM TO DRdtH

' REM

REM

REM
8 REM

"> REM -SET UP FOUR-COLOR GROPHICS--
10 G R O P H I C 3 7
20 COLOR 1

30 REM -REOD UOLUE5 INTO UORIOBLE5--
40 REOD HLEF T . XRIGHT . YTOP , YBOTTOM
50 DOTO 10.70,10,70

100 REM

101 REM --MOVE TO UPPER-LEFT CORNER--
110 PLOT HLEFT.YTOP

120 REM

121 REM -~DROW FOUR SIDES OF BOK--
130 DRAWTO KRIGHT,YTOP

140 DROWTO KRIGHT , YBOTTOM
150 DROWTO KLEFT , YBOTTOM
160 DROWTO HLEFT.YTOP

Figure 6.6; The box-drawing program, using READ and DATA statements.

USING VARIABLES 91

aind ask you to type in a value on the keyboard. With this, you will no
longer need to specify the numbers as you are writing the program.
Instead, you can supply the values at the time you RUN the pro-
gram. This also means that you can RUN the sjime program with
different values merely by giving diflferent responses.

To have the computer ask you for the vjdue of the first variable,
XLEFT, you could put this line into the box-writing program:

40 INPUT XLEFT

When you ask the computer to RUN the program, it will stop when
it comes to this statement and put a question mark on the screen.
This is a prompt to show that the computer is waiting for you to supply
some information.

Most people use a PRINT statement just before the INPUT state-
ment, so that they can know which piece of information the computer
is waiting for. I'd suggest you use something like this:

40 PRINT "LEFT EDGE";:INPUT XLEFT

If you RUN a program with this statement, the computer will pause
jind type the following message:

LEFT EDGE?

This tells you which value the computer is expecting you to supply.

Notice the two punctuation marks (;:) before the word INPUT in
this line. You learned the meaning of the colon (:) back in Chapter 3:
it separates two statements written on the saune line of a program.
The semicolon (;) serves a diflferent purpose. When you type a semi-
colon at the end of a PRINT statement, it keeps the computer from
starting a new line at the end of its message. After the end of the mes-
sage, the INPUT statement automatically types a question mark. The
resvilt will be a very readable question.

To see the INPUT statement in action, let's make yet another revi-
sion of our box-drawing program. Type the following lines:

30 REM -GET VALUES FROM KEYBOARD -

40 PRINT "LEFT EDGE";:INPUT XLEFT

50 PRINT "RIGHT EDGE";:INPUT XRIGHT

60 PRINT "TOP";:INPUT YTOP

70 PRINT "BOTTOM";:INPUTYBOTTOM

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

If you LIST the new version of the program, it should look like
Figure 6.7. Note that the variables in this program are never given
explicit values.

When you RUN this program, it will work a little diflferently from
the other versions. The computer first shifts into the four-color graph-
ics mode, foUowing the instructions in lines 10 and 20. Then, at state-
ment number 40, it prints the message LEFT EDGE? in the text
window and stops. This is your cue to type the first number — the
coordinate of the left edge of the box (XLEFT). The computer will
wait until you type a number and press RETURN. After you do
that, it will ask for three more numbers to set the coordinates of the
other three edges of the box. You might give the answers shown in
Figure 6.8.

When you're typing in your values as you RUN the progrzim, you
can use the keyboard just as you do normadly. You can correct mis-
takes with the BACK SPACE key, and you can type as many digits as
you wish. Once you press the RETURN key, however, the computer
stores your answer in its memory, and you cannot go back. Of

. REM ■■"
REM

' REM
5 REM
^ REM
10 G R i3 P H
26 COLOR
30 REM
40 PRINT
50 PRINT
60 PRINT
70 PRINT
100 REM

RAM TO DRAW

FOUR-COLOR GRAPMIC5--

, fc( JALUE5 FROM KEVB0ORD--

• I... eFT EDGE";: INPUT KLP' '
RIGHT EDGE' INPUT MRIGHT
■ M j P " . ; I N P U T Y T O P
'BOTTOM": INPUT YBOTTOM

iOl REM --MOUE TO U P P E R ^ I.. £

iiO PLOT K LEFT y TOP

120 REM

121 REM --DRAW FOUR o IDE j
130 DROWTO KRIGHT.YTOP
140 DRAWTO XRIGHT. YBOTTOM
150 DRAWTO KLEFT , YBOTTOM
160 DRAWTO XLEFT, YTOP

READY

CORNER--

Figure 6.7: The box-drawing program, using INPUT from the keyboard.

USING VARIABLES

course, you can run the prograim over again. (If you need to stop the
program before you run it over again, press the BREAK key.) Note
also that if you type anything other than a number, the computer will
object:

ERROR-- 8 AT LINE 40

When this happens, you will have to RUN the program over from
the beginning.

When you finally give your fourth response, the computer will go
on to draw the box. If you give the Jinswers shown in Figure 6.8, you
will get the same old box as in Figure 6.2. By naming other values in
response to your program's questions, you can have the computer
draw boxes of any shape.

You have accomplished two important things with this INPUT ver-
sion of the program. First, you have developed a program which asks
you questions as it works. It waits for your responses, and then acts
according to your wishes.

Figure 6.8: At each INPUT statement, the computer asks you what number
to use.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

More important, though, you now have a program which you can
RUN again and again. Each time you RUN the program it performs
the same general task, but the results vary depending on the numbers
you supply. This is an important step because your computer is now
using its program as a general problem-solver, rather than just as a set
of fixed instructions.

DOING CALCULATIONS

You have now seen three diflFerent ways you can store numbers in
variables. If you just need to assign a value to a single variable, you
can use a simple equals sign. If you have several values that you want
to store at one time, you might prefer to use the READ and DATA
statements. Finally, if you want to supply die value at die time you
RUN the program, you can use an INPUT statement.

Up to this point, however, you have been very limited in what you
could do with the numbers once you stored them. You could PRINT
them, or use them as coordinates in a PLOT or DRAWTO com-
mand, but that wais about it.

Your computer also allows you to perform calculations on the num-
bers you have stored. It can add, subtract, multiply, and divide,, and
can handle more complex mathematical functions as well. All of this is
built into the machine: you only need to tell it which formulas to use.

Your computer lets you write your formulas in a way very similar
to regular arithmetic. For the standard operations, you use die follow-
ing symbols:

+ Addition

— Subtraction

* Multiplication

/ Division

'^ Exponent ("Power")

The asterisk (*) was chosen for multiplication, since an x might be
confusing to the computer. For division die slash (/) is used, because
it resembles a fraction bar, which is a form of division. All five of these
symbols are located on or near the arrow keys at the right side of
the keyboard.

USING VARIABLES 95

You can write a formula just as you do in arithmetic. You might try
typing

ANSWER=2+2

Your computer will take this very literzilly, as a command. It takes
ANSWER as a variable and looks at the right side of the equals sign
for the value it should assign. In this case, it sees 2+2, so it calculates
the value: 4. Then, it stores this number in the variable to the left of
the equal sign: the variable ANSWER. To see that it has indeed been
stored, you can type the command

PRINT ANSWER

Your formulas can be as complicated as you wish, and can contain
variable names as well as numbers. You could type £iny of the follow-
ing lines:

X=15+17

PAY = WAGE* HOURS

AREA = LENGTH * HEIGHT/2

In the last of these examples, the formula contains more than two
numbers. As in airithmetic, the computer generally calculates formulas
from left to right, except that it gives priority to multiplication and divi-
sion operations. To see how this works, ask the computer to calculate

NUMBER = 3*4 + 6/2

The computer will calculate the multiplication (3*4) and division
(6/2) problems first. Then, it will add the £inswers together (12 plus 3)
cind store the result (15).

If you want to override this sequence, or if you simply find this con-
fusing, you can group operations together with parentheses. You
could, for instance, write the previous commcind as foUows:

NUMBER = (3* 4) + (6/2)

You will get the same zinswer. Note that you would get a difierent
answer if you grouped the operations differendy:

NUMBER = ((3*4) + 6)/2

In this case, the computer will first calculate the multiplication (3*4)
in the innermost set of parentheses, then add the answer (12) to the
number 6, to solve the outer set of parentheses. Only then will it

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

divide this result by 2 to get the final answer (9). Note that you can
put parentheses within a set of parentheses.

If you find these concepts diflScult, don't worry. I will stick to very
simple equations in this book, so that you won't get lost. If you are
comfortable with mathematics, you can certainly learn to write more
complicated formulas on your own.

You can use a formula anywhere you would use a number or a
variable. You could, for instance, type

PRINT 3*5

When you press RETURN, the computer will respond with the
answer, 15. As you recall, this is very diflFerent from

PRINT "3*5"

Within quotation marks, the computer treats 3*5 as a message to be
printed as written, not as a formula to be calculated.

As an example, let's write a program that draws a blue triangle on
the screen. (You're getting tired of boxes, aren't you?) As always,
press RESET and type NEW before you start. Then type the foUow-
ing program:

10 GRAPHICS/

20 COLOR 2:REM GREEN PAINTBRUSH

30 PRINT "STARTING X";:INPUT X

40 PRINT "STARTING Y";:INPUT Y

50 PLOTX.Y

60 REM MOVE 40 UNITS TO RIGHT

70 DRAWTOX + 40,Y

80 REM MOVE DOWN AND TO LEFT

90 DRAWTOX + 20,Y + 30

100 REM MOVE BACK TO START

110 DRAWTOX.Y

When you RUN this program, the computer will ask you for starting
X and Y coordinates. The numbers you give will define the point in
the upper-left comer of the triangle. Line 50 PLOTs this point, then
the rest of the program draws lines to the two other comers, and back
to the starting point. The result is a triangle, as shown in Figure 6.9.
RUN the program ^;ain, and give different values for the coordi-
nates. In this program, you can use any X up to 1 19, and any Y up to
49. As you try different combinations, you will find that the triangle

USING VARIABLES

always remains the same size and shape, but is moved to different posi-
tions on the graphics screen. This happens because aW of the coor-
dinates were given relative to the point in the upper-left comer of the
triangle. When you chjmge that point, the others will change with it.

STRINGS: STORING LETTERS

As a final topic in this chapter, I will describe a way to store letters
in your computer. This is a compleix subject, which could easily take
up a chapter of its own. I cannot give it that much attention, but I
can cover it briefly so that you will understand this concept when you
come across it in other books.

You can store words and letters in the computer's memory, just as
you can store numbers. To do this, you need a special kind of variable,
called a string variable. To set up a string variable, you must put a dollar
sign ($) at the end of the name. All of these are string variables:

A$
NAMES

Figure 6.9; The results of the triangle program.

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

ADDRESSS
MESSAGES

Before you can use a string variable, you must tell the computer
how long the message wiU be that it is about to store. The computer
must know this, so that it can set aside space in its memory for each
character m the string. Choose a length longer than you're going to
need, then type a statement such as this:

10 DIM NAME$(20)

DIM stands for "dimension," and this statement tells the computer to
set aside space for 20 characters in the string NAME$.

You can now store words or letters in the string variable, just as
you stored numbers in normal variables. You can use a standard
assignment statement, by enclosing the message in quotes:

20 NAMES = "ARTHUR"

You can use a READ and DATA statement:

20 READ NAMES
30 DATA ARTHUR

Or, you can use an INPUT statement:

20 PRINT "WHICH NAME";:INPUT NAMES

In the last case, the computer will ask you WHICH NAME? and
you can type ARTHUR from the keyboanJ.

When you have stored the message in the string variable, you can
use it in several ways. You can PRINT it:

40 PRINT NAMES

You can even use just part of the message by naming the starting and
ending letters of the section that you want to use:

60 PRINT NAME$(2,4)

If you RUN a program with this last statement, the computer will
display only the second, third, and fourth lettera of the name in this
case RTH.

There are many other things you can do with strings. You can
splice diem together, or pull them apart. You can display messages in
special ways on the screen, or store messages for future use. The

USING VARIABLES 99

applications of this concept are many and varied, but they are a sub-
ject for a more advanced book.

OPTIONAL EXERCISES

1 . In the version of the box-drawing program shown in Fig-
ure 6.7, add an INPUT statement before Une 20, so that
you can choose a color other than orange. You will need
to define a new variable, and will need to modify state-
ment 20.

2. Add statements to the end of the triangle program (Figure
6.9) so that the computer will draw a Star of David. You
will need to add PLOT and DRAWTO statements to
draw a second triangle on top of the first, with the point
facing upward.

3. Store the number 10 in the variable J. Then type the fol-
lowing commzmd:

J = J + 1

What vailue will J now have? Check your guess by asking
the computer to PRINT J.

SUMMARY

In this chapter, you have learned how to use variables, so that you
can store a number and reuse it later. Variables let you simplify pro-
grams, and make them perform more general operations.

The simplest way to store a number is to set the variable name
equal to it. The number will remain stored in that variable until you
replace it or turn the computer off.

If you have many numbers to store, or constantiy need to change
the values you are using, you can READ the values from a DATA
statement. Each variable in the READ statement is assigned the cor-
responding vzdue in the DATA statement. To change the values, you
need only change the DATA statement.

The most flexible way to store values is with an INPUT statement.
This tells the computer to stop each time it runs the program in order
to 2isk you to type the value at the keyboard. You type the number

100

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

and press RETURN. The computer then stores the value and con-
tinues with the program.

By writing formulas, you can have your computer do calculations
tor you. The formulas can be as simple or as complex as you wish
and you can easily PRINT the results.

You can store more than just numbers. With string variables, you can
store verbal messages as well, and PRINT them out in various ways.

In the chapters ahead, you will learn to use variables and other fea-
tures to make your computer programs even more flexible and usefiil.

B CHAPTER SEVEN

Controlling
Your Program

In the last two chapters, you have learned how to write programs.
You can now type conunands into the computer and store them as am
organized program in its memory. With vzuiables, you can make your
progrjims more flexible, so that they will perform a general task using
the values you supply.

Until now, however, you have been locked into the start-to-finish
flow of the program. The computer began to RUN the program with
the first statement, then went through each of the others in order.
When it reached the end of the program, the computer stopped and
said READY. It saw each statement only once.

In this chapter, you will learn ways to alter the flow of the program.
You can ask the computer to jump fixtm one part of the program to
another, or to return to statements it has already used. You can also
have the computer do a repetitive task many times, or make decisions
based on information you supply. Finally, you will learn how to break
your program up into smaller subprograms, which you can use as build-
ing blocks.

GOTO; JUMPING

TO ANOTHER STATEMENT

The simplest form of program control is the GOTO statement.
This simply tells the computer to jump to another statement in the
program and continue from that point. You might, for example, want
to jump straight from statement number 20 to 50, skipping any

102

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Statements in between. You could merely make line 20 a GOTO
statement:

20 GOTO 50

When the computer sees this line, it wiU immediately skip to line 50
and go on from there. Any lines that fall between 20 and 50 will never
be used, unless another GOTO statement sends the computer back
to them.

The most common use of GOTO is to send the computer back to
the beginning of a group of statements. Try the foUowing program:

10 PRINT "HELLO"
20 GOTO 10

When you RUN this, your computer will display a vertical column of
HELLOs along the left side of the screen.

The computer first encounters line 10, which tells it to display a
smgle word, HELLO. The computer then moves on to line 20.
That, however, sends it right back to line 10, so it PRINTs the word
again. When it has finished line 10 for the second time, the computer
once more proceeds to the next statement. That is line 20 again,
which, of course, sends it back to 10. The process goes on and on.

The computer doesn't even stop when it runs out of lines on the
screen. If you look carefiilly at the HELLO on the bottom line, you
will see that it is flickering. The computer is still printing the word
HELLO repeatedly, and it keeps moving all of die other lines up and
ofl" the screen to make room for it.

If you try to type a letter on the keyboard at this point, you will dis-
cover that the computer will not accept it. It is still running your pro-
gram, and all of its attention is directed towards that task. Until the
computer finishes a program and says READY, you cannot type any
new commands. This is a program, however, which will never end.

This is known as an infinite hop. The computer is doing a task over
and over again, and will continue until you turn the power off or do
something else to stop it. You can stop the program with the RESET
key, but that will also erase the screen.

The best way to stop a program is with die BREAK key, located at
the upper-right corner of the keyboard on the Atari 600XL and

CONTROLLING YOUR PROGRAM 103

800XL. When you press this key, the computer will stop in its tracks
and say

STOPPED AT LINE 10

It does not say READY, but the cursor has reappeared to show that
you can type letters again.

You will find the BREAK key very useful for stopping programs
while they are running. No matter what the computer is doing in
your progr2im, the BREAK key will stop it and let you type com-
mands from the keyboard again. This is essential, if you ever get
stuck in an infinite loop, or want to cut the program short. If you
wish, you can restjut the program from the place where you stopped
it, by typing the commzind

CONT

In most cases, though, you will probably want to use RUN to start
over from the beginning.

Infinite loops are not necessarily bad. When you v/ant to do a task
over and over, it is often easiest simply to write the progrzim as an
infinite loop and stop it manually when you have haid enough.

As an example, let's write a program that will double any number
that you type in. Try the following prograim:

10 PRINT

20 PRINT "TYPE A NUMBER";:INPUT N

30 PRINT N;" TIMES 2 = ";N*2

40 GOTO 10

The program starts by printing a blank line on the screen, to make
the display more readable. Line 20 is an INPUT statement that asks
you to type a number in from the keyboard. It waits for your
response, then puts your number in the variable N. Line 30 then dis-
plays first the number N, then the phrase "TIMES 2 = ", and then
the answer that it calculates from 2*N. The GOTO statement then
sends the computer back to the begiiming of the program, to ask you
for another number.

The PRINT statement in line 30 may look a litde unfamiliar. Up
till now, we have only used one variable or one message after the
word PRINT. Here we have three: the variable N, the message
" TIMES 2 = ", and the answer N*2. The computer understands

104

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

this perfecdy well, and will display the three items one after the other.
The semicolons (;) separating the items are important, since they tell
the computer to display the three items on the same line with no extra
spaces in between.

When you RUN this program, the computer will start by asking
you to TYPE A NUMBER. Type 3 and press RETURN. The com-
puter will respond with the answer:

3 TIMES 2 = 6

It then goes on and asks you to type cinother number. You can type
numbers and have the computer double them as many times as you
wish. Figure 7.1 shows how your screen might look after a few more
responses. When you're done, press BREAK to regain control of the
keyboard.

You can make any program repeat in this way. A good exzimple
might be the box-drawing program we developed in Chapter 6.

«Eaov

NEH

« e ft D Y

10 PRINT
20 PRINT
30 PRINT m
40 GOTO 10
RUN

PE C> NUMBER'

TIMES 2 15

; ; INPUT

" ; H*2

TYPE ft N U MB E R ' 3
S TIMES 2 IS 6

TYPE ft NUMBER^23
23 TIMES 2 IS 4&

TYPE a MUMBER-'9354
9354 TIMES 2 IS 18708

TYPE ft NUMBER'>0
TIMES 2 XS O

TYPE ft NUMBER''

Figure 7.1: With a GOTO statement, you can have the computer do a task
many times.

CONTROLLING YOUR PROGRAM

105

Think about what would happen if you added the following line to the
program in Figure 6.7:

170 GOTO 30

The program would RUN as usual, asking you to INPUT the coordi-
nates for the comers of the box. Once you'd typed in four numbers,
the computer would immediately draw a yellow box.

Unlike the progrzim in Figure 6.7, however, the computer wouldn't
stop when it had finished. Instead, it would go back and ask you for a
new set of four numbers. If you typed four more coordinates for the
comers, the computer would draw a second box without erasing the
first. You could draw as many boxes as you like. Figure 7.2 shows
what your screen might look like after four times around the loop.

If you look carefully at the program listing in Figure 6.7, you will
notice that this GOTO statement does not send the computer all the
way back to the beginning of the program. This is because it is not
necessary to repeat the GRAPHICS and COLOR statements in lines

Figure 7.2: You can modify the box-drawing program so that you can draw
more than one box on the screen.

106 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

10 and 20. In fact, the GRAPHICS statement would have cleared the
screen each time around and erased die picture we were in the process
of building. By using GOTO 30, we repeated only those lines that
were necessauy.

The GOTO statement is so convenient that many people overuse
it. Programs that jump back and forth too much can be very diflScult
to follow, so you should use this statement only when tiiere is a dear
need. People normally expect to read a program from top to bottom,
and too many departures from diat order can be confusing.

FOR/NEXT LOOPS

With the GOTO statement, you have learned how to make the
computer repeat a part of your program. The problem is that all of
your loops have been infinite: the computer simply repeated the pro-
gram until you pressed the BREAK key to make it stop.

Often, however, you want to have the computer repeat a group of
statements a fixed number of times, then go on to another part of
the program. To do this, you must use two new statements: FOR
and NEXT.

They work like this. You put the FOR statement at the beginning
of the lines you want to repeat, then type a NEXT statement at the
end. When the computer comes to the FOR statement, it starts up a
counter to keep track of the number of repetitions it has performed. It
then keeps repeating all of the statements between the FOR and the
NEXT until it reaches the limit you have set. After that it does not
jump back when it reaches the NEXT statement, but passes through
and continues with the rest of the prograim.

To see this in action, try the foUowing program:

10 PRINT "THE LOOP IS BEGINNING."

20 F0RCNTR=1 TO 10

30 PRINT "REPETITION NO. "; CNTR

40 NEXT CNTR

50 PRINT "THE LOOP IS FINISHED."

The messages in this program's PRINT statements are arbitrary-
chosen for the purposes of explanation. CNTR is the name chosen
for the counter variable, with which the computer keeps track of the

CONTROLLING YOUR PROGRAM

107

repetitions. When you run the program, your results should look like
Figure 7.3.

The FOR statement in line 20 tells the computer to repeat the loop
a number of times, keeping track with the counter variable nzimed
CNTR. The '1 TO 10' in the FOR statement sets the starting and
ending values of the counter, and therefore also the number of times
to repeat the loop. As you can see from the results of your program,
the counter variable starts at 1 on the first pass through the loop, and
increases by one each time through. When it reaches the upper limit
(10), the counter tells the computer that the loop is done, and the
computer proceeds to the statement following the NEXT.

Line 30 shows a useful feature of FOR/NEXT loops:

30 PRINT "REPETITION NO.";CNTR

Within the loop, you can use the counter as a normal variable. Each
time through, CNTR will contain a diflFerent value, depending on the
number of times the computer has repeated the loop. With careful
plarming, you can often use this to your advantage.

10 PRINT "THE LOOP 15 BEGINHIMG." |

2eF0RC0UMTER3:iT0i0

36 PRINT 'REPETITION NO.";COUNTER

40 NEXT COUNTER

56 PRIN^ THE LOOP IS FINISHED."

REftDV

RUN

THE LOOf- i> BEGINNING

REPETITION NO 1

REPETITION NO . 2

REPETITION NO . 3

REPETITION NO . 4

REPETITION NO . 5

REPETITION NO . 6

REPETITION NO . 7

REPETITION NO . 8

REPETITION NO . 9

REPETITION NO . iO

THE LOOP IS FINISHED.

READV

■

Figure 7.3: An example of how a FOR/NEXT loop can be used to repeat a
PRINT statement.

108 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

The counter variable does not need to start from 1. You can choose
any numbers you want for the starting and ending values— even neg-
ative numbers. Try some other values in line 20 of this program, such
as the following:

20 F0RCNTR = 7T0 14

20 FORCNTR = 329T0 364

20 FORCNTR=-3T0 6

20 FORCNTR = 8T0 5

Note that in the last of these examples, the upper limit on the counter
was below the starting value. When this happens, the computer goes
through the loop only once, then continues past die NEXT statement.
You can also ask the computer to increase the counter each time by
some number other than 1 . You do this by adding zmother number to
the end of the FOR statement, like this:

20 FOR CNTR = 1 TO 1 STEP 2

If you RUN the program with this statement, you will find that the
counter skips by twos: 1, 3, 5, 7, 9. Note that the counter never
reaches exacdy 10 in this czise, but stops the loop at 9, just before it
goes over the limit.

You can also use the STEP to make the loop counter go down
instead of up. To do this, give die STEP a negative value:

20 FOR CNTR = 1 TO 1 STEP - 1

If you run the program in this way, the counter will start with the
value 10. When it reaches the end of the loop, it will add - 1 to the
counter, eflfectively subtracting one from it. The loop will then repeat
with the values 9, 8, and so forth. When it reaches 1, the loop will
end and the program will continue.

You can give the counter variable ziny name you want. You must
make sure, however, that you make the name in the FOR statement
exacdy match die name you use in the NEXT statement to end the
loop. If you don't, you get an ERROR. Many people choose to give
simple names to their FOR/NEXT counters— especially one-letter
names such as I, J, and K.

CONTROLLING YOUR PROGRAM

109

You can have any group of statements within your loop — even
another loop. Try out the following progreim:

10 PRINT "THE LOOPS ARE BEGINNING"

20 FOR I = 1 TO 3

30 PRINT "OUTER LOOP --" ;l

40 FOR J = 1 TO 4

50 PRINT "INNER LOOP --" ;J

60 NEXT J

70 PRINT "INNER LOOP IS FINISHED"

80 NEXT I

90 PRINT "BOTH LOOPS ARE FINISHED"

The results of this program are shown in Figure 7.4. As you can see,
the outer loop is repeated three times in the course of the program,
each time printing the value of its counter variable, I. Each time
through this outer loop, however, the computer encounters the inner
loop, with a different counter variable, J. This tells it to repeat the
PRINT statement in line 50 four times on each of its three passes
through the outer loop.

RUM 1

THE LOOPS ARE BEGINNING

OUTER LOOP -- i

INNER LOOP - 1

INNER LOOP - 2

INNER LOOP

INNER LOOP 4

INNER LOOP IS FINrSHFD

OUTER LOOP

INNER LOOP 1

INNER LOOP

INNER LOOP '

INNER LOOP 4

INNER LOOP r. FINI, HtL-

OUTER LOOP i

INNER LOOP 1

INNER LOOP

INNER LOOP J

INNER LOOP 4

INNER LOOP 15 FINISHED

BOTH LOOPS ARE FINISHED

READY

■

Figure 7.4: A loop within a loop can give interesting results.

110 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

There is only one restriction on this useful idea of a loop within a
loop: the inner loop must be entirely contained within the outer, like
the pieces of a Russian doll. You must make very sure that the NEXT
of the inner loop comes before the NEXT of the outer loop, as in the
above program. Otherwise you will get an ERROR.

FOR/NEXT loops are useful for almost every type of program.
Most progrzims need to repeat statements in some way, and for many
tasks it is essential. Here are some examples of ways you can use
FOR/NEXT loops:

Calculations Many computations have to be done over and over
again. Often, you need to have a whole list of answers to the same
simple problem, and you'd rather not have to calculate it each time.
Suppose, for instance, that you wanted to buy a number of gadgets
at $11.39 apiece. You'd like to buy several, but the number you buy
will depend on the cost. So, you write a program to make up a table:

10 REM * * * TABLE OF GADGET COSTS* * *

20 PRICE= 11.39

30 REM PRINT HEADINGS

40 PRINT "NUMBER", "COST"

50 FOR I = 1 TO 20

60 COST = l*PRICE

70 PRINT I.COST

80 NEXT I

When you RUN this program, you will get a table, as in Figure 7.5.
This shows how much you would have to pay for any number of gad-
gets from 1 to 20. Notice that in lines 40 and 70 a comma was used
instead of a semicolon. A comma between PRINT messages teUs the
computer to arrange numbers neady in columns.

Graphics For many types of pictures, you must use some controlled
form of repetition, so that you don't have to plot every point and line
manually. Any figure that cannot be made up of a few straight lines
must be drawn using a FOR/NEXT loop.

You will need to use a FOR/NEXT loop if you want to draw a
solid figure. To see how this works, try the following progrjim:

10 REM * * *SOLID TRIANGLE * * *
20 GRAPHICS?

CONTROLLING YOUR PROGRAM

111

30 COLOR 1

40 FORY = 0TO79

50 PLOTO.Y

60 DRAWTOY.Y

70 NEXTY

This program steps through every possible value of Y, from at the
top of the screen to 79 at the bottom. At each Y, it PLOTs a point at
the left edge of the screen (coordinate 0,Y), then draws a horizontal
line Y units long. The result is a solid triangular region, as shown in
Figure 7.6.

Delays Each time the computer has to repeat a FOR/NEXT loop, it
wastes a fraction of a second. In most programs, you will hardly
notice this delay. If you ask for a large number of repetitions, though,
the time begins to add up.

You can use this to your advantage when you want to slow down

Figure 7.5: You can create a table with a FOR/NEXT loop.

112 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the action or have the computer pause. Write a FOR/NEXT loop
with many repetitions at the place where you want the pause:

20 FOR DELAY = 1 TO 5000
30 NEXT DELAY

This loop does nothing whatsoever as it goes through its 5000 repeti-
tions; however, it wastes about ten seconds in the process. This might
be helpful in a program such as this:

10 PRINT "PATIENCE TEST"

20 FOR DELAY = 1 TO 5000

30 NEXT DELAY

40 PRINT "YOU PASSED!"

When you RUN this program, the computer immediately displays
the words

PATIENCE TEST

It then goes into the "do-nothing loop" and waits about 10 seconds
before congratxilating you.

Figure 7.6: You can draw solid regions on the screen, using FOR/NEXT loops.

CONTROLLING YOUR PROGRAM 113

Delay loops are common in many Atari programs. Since the FOR
and NEXT statements here are redly a single idea, many people
write them on a single line, separated by a colon:

20 FOR DELAY = 1 TO 5000: NEXT DELAY

In these and other uses, FOR/NEXT loops give you a new range
of control over your computer They let you repeat a group of state-
ments 2iny number of times, then go on to the next part of the pro-
gram. While the computer is repeating the loop, it also keeps track of
a counter variable, which you can use in your calculations. You will
find more and more uses for these statements as you explore the capa-
bilities of your computer.

IF STATEMENTS:

MAKING DECISIONS

You can also control the way the computer runs your program by
having it make decisions. With the IF statement, you can have the
machine decide whether or not to do something depending on the val-
ues of certain variables.

Of course, the computer isn't meiking an independent decision.
You tell it to do one thing if a certain condition is met, and another
thing if it isn't. It makes its choice in response to the information you
give it. The computer doesn't really think; it just follows the course
that fits the conditions.

The IF statement consists of two parts: a condition and a response.
The two parts of the statement are signalled by the words IF and
THEN. The general form of the statement is

10 IF cond/ton THEN response

When you actually write this statement, you will replace the words
condition cind response with a specific comparison and command.

The condition vwll usually be a comparison between a variable zmd
some numbers. You could, for example, test to see if the variable
NUMBER is equal to 0. You would stzirt by writing

10 IFNUMBER = OTHEN . . .

The computer would check the vedue stored in NUMBER. If it is
exactly 0, the computer will cany out the response. If it is not, the

114 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

computer will skip the response and proceed to the next statement in
the program.

You can use a variety of arithmetic comparisons in your IF condi-
tion. The most important are

= equal to

> greater than

> = greater than or equal to

< less than

< = less than or equal to

< > not equal to

You can also combine two or more conditions with the words AND
and OR. If you wanted to see if a number was more than 5 and less
thjin 10, you could use the condition

10 IFNUMBER>5ANDNUMBER<10THEN . . .

The computer would carry out the response only if the variable
^fUMBER had a value between 5 and 10.

The response, which follows the word THEN in the statement, can
be any Atari command. If the condition in the IF part is satisfied, the
computer will foDow the command; if the condition isn't satisfied, it
will pass the command by.

Here are some examples of valid IF statements:

10 IF 1 = 10 THEN PRINT "NUMBER IS TEN"
20 IF MODE = THEN GRAPHICS
30 IF SIZE >20 THEN SIZE = 20

Line 10 will display a message on the screen if at that point the vari-
able I is equal to ten. If I has any other value, no message is dis-
played. Line 20 uses an IF statement to reset the computer to
GRAPHICS mode (standard text), if it discovers the variable
MODE has been given the value 0.

Line 30 shows a common use of the IF statement. When it arrives
at this statement, the computer checks to see if the veiriable SIZE has
exceeded 20. If it has, the computer resets it to 20. After this state-
ment, you can be certain that SIZE will not be greater than this maxi-
mum value.

CONTROLLING YOUR PROGRAM

To see the IF statement in action, let's try a simple program that
lets you guess at a number from 1 to 100. Type in this prograim:

10 ANSWER = 57

20 PRINT

30 REM BLANK LINE

40 PRINT "TYPE A NUMBER FROM 1 TO 100"

50 INPUT N

60 IF N< ANSWER THEN PRINT "TOO SMALL"

70 IF N>ANSWER THEN PRINT "TOO BIG"

80 IF N = ANSWER THEN PRINT "CORRECT!"

90 GOTO 20

Line 10 of this program gives the correct Jinswer to the computer. The
rest of the progrzim is am infinite loop that asks you to type a number,
then compares it to the correct answer.

When you RUN this program, the computer will start by asking
you to

TYPE A NUMBER FROM 1 TO 100

You know the answer (57), but try some other numbers too. Give your
response and press RETURN. The computer will say either TOO
SMALL, TOO LARGE, or CORRECT, depending on the nimiber
that you type. The computer makes its choice according to the IF state-
ments in lines 60 throu^ 80, then PRINTs the appropriate response.
Line 90 sends the computer back to the beginning of the loop, to ask
for another number. Figure 7.7 shows the results of several guesses.
When you have finished, press BREAK to stop the program.

You can make the computer expect a different answer, merely by
changing line 10. You might even enjoy playing this as a game. Store
a number in the computer by typing it into line 10, then dear the
screen and RUN the program. You can then have a friend try to
guess the number that you typed in.

IF statements are often used to ask the computer to CJOTO another
part of the program only under a certain condition. You could, for
example, have the ntmiber-guessing program go back to line 20 only
if the answer v/as wrong. Replace the simple GKDTO statement in line
90 with the following IF statement:

90 IF N < >ANSWER THEN GOTO 20

115

116

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

The <> is a "not equals" condition, and will send the computer on
two different paths through the progrcim, depending on whether the
values of the variables N and ANSWER are equal or not. If they are
not equal, the computer will jump direcdy back to line 20 and con-
tinue from there. If the numbers are equal, however, the condition
will be false, and the computer will skip the GOTO command. Nor-
mally, it would then continue with the next statement, but since this is
the last line, the program simply ends.

IF statements with a GOTO response allow you to split your pro-
gram into two parts, then let the computer choose which path to take,
depending on the circumstances. You might, for example, write a
prograim such as this:

10 PRINT "TYPE A NUMBER";:INPUT N

20 IF N = THEN GOTO 50

30 PRINT "STATEMENT 30 IS USED"

40 GOTO 10

50 PRINT "STATEMENT 50 IS USED"

60 GOTO 10

■■■

■

RUN

TVPE Ct NUMBER

'35

TOO 5M0LL

FROM 1

100

TYPE a NUMBER
?64

TOO BIG

FROM 1

100

TVPE a NUMBER

?51

TOO 5MOLL

FROM 1

1O0

TYPE O NUMBER

?58

TOO BIG

FROM 1

lOO

TYPE 6 NUMBER

?S7

CORRECT '

FROM X

10©

TYPE 6 NUMBER

FROM 1

100

I^HI

■

Figure 7.7: Results of the number-guessing program.

CONTROLLING YOUR PROGRAM 117

This program asks for a number, then tests to see if it is zero. If it is,
the computer jumps to statement 50 and prints a message. If the num-
ber is not zero, the computer bypasses the response and continues with
the next statement, number 30. In either case, the computer returns to
line 10 after it PRINTs the message.

The IF statement lets you use a technique called data checking.
Often when you write a program that asks for input from the key-
bojird, you are expecting a response of a certain kind. You might, for
instance, expect a number from 1 to 12, to represent a month of the
year. You can guard against meaningless responses by making a
simple check:

10 PRINT "WHICH MONTH";:INPUT MONTH

20 IF MONTH <1 OR MONTH >1 2 THEN GOTO 10

30 PRINT "OK"

When you RUN this program, the computer will ask you to type
a number to represent a month. If the value is meaningless, however,
the IF statement will send the computer back to line 10 to ask
for another value. It will keep doing this until you type a number
between 1 and 12. In any statements you might add after line 30,
you can be sure that the variable has a value that represents an
actuzd month.

IF statements dezirly have many other uses. Whenever you want
your computer to do more than a simple task, you will need to have it
make decisions along the way. With the IF statement, you can ejisily
control the way it makes these choices.

GOSUB: SUBPROGRAMS

There is another important way you can ch£inge the start-to-finish
flow of your program. This is with subprograms, a series of statements
which you can set oflF from the rest of your program and use as a unit
whenever you want.

To use a subprogram, you must call it with a GOSUB statement,
such as this:

40 GOSUB 1000

This acts just like a GOTO statement, sending the computer to line
number 1000 to continue its work. With GOSUB, however, you tell

118

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the computer that you want it to come back after it has finished using
the subprogram.

The subprogram itself is like any other group of statements, except
that it ends with the command RETURN. When the computer gets
to this final command, it sees that die subprogram is done, and goes
back to the statement following the original GOSUB. In diis way, you
can tell die computer to go off and follow a group of instructions, then
come back to the point where it left oflF.

To see how this works, let's try a sample program:

10 PRINT "MAIN PROGRAM BEGINS"

20 GOSUB 1000

30 PRINT "MAIN PROGRAM CONTINUES"

40 END

1000 REM SAMPLE SUBPROGRAM

1010 PRINT "NOW RUNNING SUBPROGRAM"

1020 RETURN

When you RUN this program, the computer will respond like diis:

MAIN PROGRAM BEGINS
NOW RUNNING SUBPROGRAM
MAIN PROGRAM CONTINUES

From this you can trace the flow of the program. The computer
began with line 10, die beginning of the main program. At line 20,
it jumped to the subprogram and printed the second message. At
die RETURN statement, however, die computer finished die subpro-
gram and went back to the main program, continuing with line
30, which immediately follows die GOSUB statement diat called die
subprogram.

The END command in line 40 is quite important. It signals the
end of die main program and tells die computer to stop. If this state-
ment were not there, the computer would continue running the pro-
gram, going on to line 1000 of die subprogram, which would be die
next line. This would not be what you wanted, unless you did in fact
want it to rerun the subprogram at the end of the main one.

You can have many different subprograms, as long as each has
different statement numbers and ends with a RETURN. While you
are free to choose any statement numbers for your subprograms, you
will find it best to use radier high numbers, such as die 1000 chosen
here. In diat way you leave space in case you later decide to expand

CONTROLLING YOUR PROGRAM

your main program. Also, it is generzilly best to start each subpro-
gram with a remark (REM statemem), that describes what the sub-
program does.

You can put any kind of statement in a subprogram. You can even
use another GOSUB statement to call a second subprogram. If you
do this, the RETURN statement from the second subprogram will
send the computer back to the GOSUB statement in the first. The
computer will go on from there to complete the first subprogram, then
return to finish up the main program.

You may be tempted to use a GOTO radier than a RETURN to
get back to the main program. Don't. There is a way you can do this,
but you will usually end up confusing both yourself and your com-
puter. Make your subprograms self-contained, and be careful to have
them RETURN when they are done.

Why use subprograms? There are a variety of reasons why you
might want to break up your program into smaller parts. Here are
a few:

• To use a group of statements more than once. In large programs,
you often have certain tasks which you need to do over
and over. If you just want to repeat the group a number of
times in one place, you might be able to use a FOR/
NEXT loop. If, however, you need to reuse the group at
another part of your program, you would need to type the
FOR/NEXT loop over again at each point where you
want to use it. Instead of doing this, you can group the
statements together in a subprogram. Then, each time
you need to do the task, you can call it with a simple
GOSUB statement.

• To make a program modular. Very long programs can seem
terribly daunting if you try to view them all at once. By
breaking the program down into more manageable units,
you can make it much easier to understEmd. Some people
write their main programs as a series of GOSUB state-
ments, which direct the computer to each of the modules
in turn.

• To develop a subprogram library. At some point, you might
want to put together a collection of subprograms that you
often use as building blocks for different kinds of progrzims.

119

120

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

If you store all of these routines on a cassette or diskette,
you can load any one of them back into the computer
whenever you want to use it in a program.

With subprograms, we have reached the end of our exploration of
the statements that control the flow of your program. With die state-
ments you have learned in this chapter, you are well on your way
toward programming your computer eflFectively

OPTIONAL EXERCISES

1. Write a program that will ask for a number, then say
whether it was positive, negative, or zero. You will need
an INPUT and an IF statement. Remember that you use
a minus sign to type a negative number.

2. Write a subprogram that dears the screen by PRINTing
ESC and CONTROL/CLEAR. Then tack it on to one of
your other programs to improve the appearance of the
screen display.

3. Write a FOR/NEXT loop to calculate the sum of the
numbers from 1 to 100. Hint: set up a variable SUM and
add the counter variable to it each time through the loop.

SUMMARY

In this chapter, you have learned many new ways to control your
computer. You are no longer locked into a simple top-to-bottom
march through your program; you can jump around, turn loops,
make decisions, and run subprograms.

The GOTO statement tells the computer to jump to another state-
ment. You can jump forward or backward within your program, and
create infinite loops if you wish. This is often a simple way to do
repetitive tasks.

With FOR and NEXT statements, you can make your computer
repeat a group of statements a certain number of times, then go on to
the rest of the program.

IF statements let your computer make decisions in the course of
your program. At any point, you can teU the computer to test a con-
dition. If the condition is true, your IF statement will have the

CONTROLLING YOUR PROGRAM 121

computer take some action. It can print a message, change a value, or
go to zmother part of the program — anything you choose.

The GKDSUB statement gives you yet another way to determine the
flow of your program. You can break a Ijirge program up into smaller
blocks, then use the blocks whenever you need them. This lets you
write simpler progrzmis, which you can work with one piece at a time.

With this chapter, you have come to the end of the standard pro-
gramming techniques covered in this book. Chapter 8 is a whirlwind
tour of the advainced graphics features of your Atari computer, which
will show you some more of its potentiad.

B CHAPTER EIGHT

Advanced
Graphics

In Chapter 4, you learned a little about the Atari graphics system.
You learned how to use the POSITION statement to place text mes-
sages anywhere on the screen. You found out how to plot points and
draw lines in the four-color graphics mode, creating pictures with
your computer.

In this chapter, you will leaim more about the Atari graphics sys-
tem. You will learn how to control the colors of the screen baickground
and the graphics paintbrushes. You will find out how to eliminate the
text window at the bottom of the screen, so you can draw larger pic-
tures. Finally, in a tour of the other graphics modes, you will discover
other ways to control the screen display.

In these few pages, I cannot describe the entire Atari graphics sys-
tem. Instead, I have maide this a whirlwind tour to give at least a taste
of some of the wonderfvil things you can do with your Atari graphics.
If you would like a more complete description, consult one of the
more technical books devoted to the Atari.

This chapter deals with difficult subjects. The deeper points of the
Atari graphics system are very interesting, but they can be confusing
at first. Don't worry if you get hung up from time to time. Just go on
to the rest of the book, and come back when you feel you understand
better.

CHANGING COLORS WITH SETCOLOR ^

So far, you have used only a few colors: the white and blue letters
and background, amd the four graphics paintbrushes (orange, green,
blue, and black). Your computer has a range of 16 colors to choose

124

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

from. Within each of the 16 colors, you can select 16 luminances, or
shades. So in effect, you have 256 colors at your disposal. Even if you
are using a black-and-white television, you wiU be able to control the
brightness of the gray tones.

You have been using the COLOR command to choose among the
four graphics paintbrushes, but you have not been able to change the
preset color that was assigned to each one (orange, green, blue, and
black). You could choose your colors to some extent by switching
paintbrushes, but you did not have full control.

The COLOR command has a companion called SETCOLOR,
which assigns any colors you choose to die four paintbrushes. You
could, for example, make the first paintbrush white instead of orange.
While you cannot use more than four colors at one time, you can
choose those four from any of die 256 possibUities available.

Don't confuse COLOR with SETCOLOR. COLOR chooses
which of the four paintbrushes you will be using. So when you ask for
COLOR 2, you are reaUy asking for paintbrush 2. You can think of it
as saying, "Pick up diis paintbrush and keep using it untU I tell you
otherwise." SETCOLOR, on the other hand, sets die color of paint
that will be found on each paintbrush. It says, in effect, "Wash out
dus paintbrush and dip it in a new color." Unlike an ordinary paint-
brush, however, SETCOLOR also goes back and changes the color of
all die points diat brush has aL^ady painted on the screen.

The SETCOLOR command always contains diree numbers, m a
form such as this:

SETCOLOR 2,9,4

Each of the diree numbers diat follows die word SETCOLOR has a
special meaning. Figure 8.1 gives a quick-reference chart for using die
diree numbers in die four-color graphics mode.

The first number (A) names die color register you want to change. A
color register is a place where die computer can store a code diat con-
trols one of the colors on the screen. Once stored, the new code
changes die color of die paintbrush attached to diat register, widiout
affecting any of the others.

The Atari computers have five color registers, numbered dirough
4. Unfortunately, die registers have different meanings, depending on
which graphics mode you are using. For instance. Register 2 sets die
color of a paintbrush in four-color graphics, but in text mode it

ADVANCED GRAPHICS

125

merely controls the background. It would be quite difficult to learn the
meanings for all of the modes, but you can easily master the most
important ones.

The first column of Figure 8.1 shows the meaning of each of the
registers in the four-color graphics mode. Note that the registers in the
SETGOLOR command are numbered differently from the "paint-
brush numbers" you have been using in your COLOR statements.
(This may seem confusing, but you caji understand it if you remem-
ber that the "erasebrush" is always in the COLOR statement, and
always the last of the four SETCOLOR registers number 4.) Figure
8.2 shows the correspondence of the COLOR and SETCOLOR
codes, along with the preset colors of the five registers. Just find the
paintbrush you want in the COLOR column, then use the corres-
ponding SETCOLOR register to change it. Register 3 (red) is not
used in this mode, but is important for some of the others. The preset
colors of the registers are the same in every mode.

The second number in your SETCOLOR command controls the

SETCOLOR A, B, C

A: COLOR REGISTER

B: COLOR (HUE)

C: LUMINANCE

0— Paintbrush #1

0-Gray

0— Very dim

1— Paintbrush #2

1 —Light orange (gold)

2- i
4-

6-.. J-

2-Paintbrush #3

2— Orange

3— (unused)

3— Red-orange

4-Erasebrush (#0)

4- Pink

g_Medium

and background

5— Pink-purple

10-

6— Purple-blue

12- i

7- Blue

14 — Very bright

8-Light blue

9— Blue-green

10— Turquoise

11— Green-blue

12— Green

13- Yellow-green

14— Orange-green

15- Light orange

Figure 8.1: The codes for the three numbers in the SETCOLOR command in the
four<olor graphics mode.

126

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

color. In this mode you may choose from the sixteen colors shown in
column B of Figure 8.1. You will notice that the colors are arranged
like an endless rainbow. If you move through die colors from to 15
and back to 0, you will see diem go through an almost continuous
cycle. This is often useful in programs.

The third number in die SETCOLOR command sets die lurmnmce,
or brightness of the color. For any of the sixteen colors, you can
choose from eight different shades. The lowest, 0, is almost black,
while die highest, 14, is very bright. In between, you can adjust die
brightness to fit your needs. Note diat in diis mode your computer
accepts only even numbers for die luminance setting. If you use an
odd number, it will be rounded down.

To see how all diis works in practice, let's change the colors of die
paintbrushes. To get some lines painted widi die diree paintbrushes,
type the following program into your computer:

10 GRAPHICS 7

20 COLOR 1

30 FORY=10TO20

40 PLOT20,Y

50 DRAWTO 140,Y

60 NEXTY

70 COLOR 2

80 FORY = 30TO40

90 PLOT20,Y

100 DRAWTO 140,Y

110 NEXTY

120 COLORS

130 FORY = 50TO60

140 PLOT20,Y

150 DRAWTO 140,Y

160 NEXTY

When you RUN diis program, your computer will shift into die four-
color graphics mode and draw diree wide horizontal bands on the
screen. Each band is drawn widi a different paintbrush in its preset
color: one orange, one green, and one blue. In the text window at
die bottom of die screen, die computer says READY to show diat it
has finished.

Now diat you have regions painted widi each of die diree brushes,
you can change their color settings. Let's first change paintbrush 1

ADVANCED GRAPHICS

127

from orange to some other color. Type the following command;
SETCOLOR 0,5,12

When you press RETURN, the orange bar will turn to a bright pink.
The first number names the register you are changing: register 0,
which controls paintbrush number 1. The other two numbers name
the color and the brightness you are changing that register to; color 5
(pink-purple), with a brightness of 12,

Try changing either the color or the brightness of this first bar,
using other SETCOLOR commands. Also, change the colors of the
other two paintbrushes with SETCOLOR 1 and SETCOLOR 2
commands. Don't be surprised if some of these commands also affect
the text window. In changing paintbrushes, you are also changing the
text and background colors of the bottom of the screen. There is no
way to change one without the other.

You can change the color of the screen background by changing the
SETCOLOR 4 register. Try typing this, for example:

SETCOLOR 4,5,6

The entire background of the screen will change to pink. This
changes the color drawn by the erasebrush, paintbrush 0, to pink as
well, since erasure is simply painting over with the background color.
If you get tired of your new colors, you can return the machine to its
preset colors. Use the values shown in the right column of Figure 8.2:

SETCOLOR 0,2,8
SETCOLOR 1,12,10

SETCOLOR
REGISTER

PAINTBRUSH
NUMBER
(COLOR)

PRESET
COLOR

SETCOLOR
CODE

1
2
3

COLOR 1
COLOR 2
COLOR 3

COLOR
(erase)

Orange

Green

Blue

Pink or Red

Black

0,2,8

1,12,10

2,9,4

3,4,6

4,0,0

Figure 8.2: The COLOR and SETCOLOR codes in the four<ohr
graphics mode.

128

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

SETCOLOR 2,9,4
SETCOLOR 4,0,0

These are called the defauU values, because they are the codes that arc
stored in the color registers when you turn your machine on. You can
get back to these codes by pressing RESET as well, though that will
also clear the screen.

You can also use SETCOLOR to change the appearance of the
screen in the regular text mode (GRAPHICS 0). The SETCOLOR
registers have diflFerent meanings in this mode, but they are no more
difficult. Instead of controlling paintbrushes, the registers in the text
mode control regions of the screen — text, background, and border.
Figure 8.3 shows their meanings.

Register 1 controls only the luminance of the letters, not their color.
The letters are always the same color as the background; you can set
them Ughter or darker, but never to a diflFerent hue.

To see how this works, press RESET to return the color registers to
their original state. The default setting of the background color,
shown in Figure 8.3, is color 9 with a brightness of 4— dark blue. The
letters of the word READY are set by register 1 , which stsrts out as
color 12 and brightness 10, which would normally be light green. The
computer, however, ignores the color and uses only the brightness
(10), making the letters light blue, rather than green. When the letters
have a brightness less than the screen background, they show up dark
against a lighter background.

Try changing the colors of the letters, background, and screen bor-
der. Here are some interesting combinations:

SETCOLOR 2,0,0
SETCOLOR 2,15,14

SETCOLOR
REGISTER

REGION
OF SCREEN

PRESET
COLOR

SETCOLOR
CODE

1
2
3
4

Text Luminance
Background

Border

Orange
Green

Blue

Pink or Red

Black

0,2,8

1,12,10
2,9,4
3,4,6
4,0,0

figure 8.3; The SilCOlOR registers in the text mode.

ADVANCED GRAPHICS

129

SETCOLOR 2,9,4
SETCOLOR 1,0,14
SETCOLOR 1 ,0,0
SETCOLOR 4,5,10

You can always press RESET to return to the blue screen.

If, at any point, you change the letters and the screen background
so that they have the same brightness value, the letters will seem to
disappear. They are still there, but you carmot distinguish them from
the screen around them. To get them back, press RESET or blindly
type another SETCOLOR command.

ELIMINATING THE TEXT WINDOW

In all of your graphics programs, a blue window has remained for
text at the bottom of the screen. This is important, because if the
computer used the whole screen for graphics, you would have no
place to type your commands.

At times, however, you will wamt to draw pictures that cover the
entire screen. Full-screen pictures are generally more attractive, since
they are not disrupted by the blue region. Also, some of the more
advanced graphics modes cannot be used with a text window.

You ask the computer to remove the text window simply by adding
16 to the mode number in the GRAPHICS statement:

10 GRAPHICS 7 + 16

Since 7+16 equals 23, you could also write this:

10 GRAPHICS 23

It is better to write 7+16, though, because this makes it clearer what
you are doing.

Since the graphics will now extend below the bottom of the regular
graphics screen, you can use some coordinates that are larger than
normal. The limits on x are the same as before (0 to 159), but the y-
coordinates can now run as high as 95 (rather than the usual 79).

Since these extended graphics screens leave you no space to type
commands, you can use them only in programs. You must write com-
mands into your program for everything you want the computer to
draw, then type the RUN command. Once the computer switches into

130 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

this kmd of graphics mode, you can no longer give dirm commands.
It you let your program end normally, the computer will not leave
your picture on the screen, as it usuaUy does. Instead, it will erase the
entire picture immediately, to make rx)om for the word READY. To
prevent this, send the computer into an infinite loop with the last
statement m the program, so that it draws the picture and then gets
stuck. You do this with a statement such as this:
999 GOTO 999

When die computer gets to this final statement, it will simply stop
with your picture frozen on the screen.

The computer still won't respond to what you type on the key-
board: It is busy repeating line 999. When you're ready to give com-
mands again, press the BREAK key to stop the program. This will
erase the picture from the screen and let you type commands again.

Iry the foUowmg program to see how full-screen graphics work:

10 GRAPHICS 7 + 16

20 COLOR 1

30 PLOT 0,0

40 DRAWTO 159,95

50 PLOT 0,95

60 DRAWTO 159,0

999 GOTO 999

When you RUN the program, the computer should draw an orange X
across die entire screen. Line 999 locks it in an infinite loop, so that die
picture wdl remain on the screen. Press BREAK to regain control

THE GRAPHICS MODES ■

So faryou have seen only two of your computer's sixteen graphics
modes. These are die most important, but die odier fourteen are also
useful. By usmg die mode diat exacdy suits your puiposes, you can
design your graphic displays just die way you want diem.

Figure 8.4 is a comparison chart diat shows what each of die modes
does. Look carefully at die diflferences between die modes.

The resolution column is very important. This pair of numbers tells
how many rows and columns die screen is divided into. The higher
die resolution, die finer your points and lines can be. If you want to
draw very detailed pictures, dien you wiU want die highest possible

ADVANCED GRAPHICS

131

resolution. On the other hand, if you want to draw pictures with thick
lines, you can use lower resolution modes.

The resolution you choose also affects the amount of memory you
use. In high- resolution graphics modes, the computer must use sub-
stantial memory space to store the information it needs to paint the
picture. This is very important if you have an Atari 600XL without
memory expansions, because some of the modes will lake up almost
the entire available memory. This can cause problems, because in
these modes you may not be able to write programs longer than ten
or fifteen lines. You can use all of the graphics modes on your
machine, but you must be very careful with modes that use more
than 8000 bytes of memory (see the right column of Figure 8.4).
Memory is not a problem if you have a memory expansion or one of
the more expensive Atari computers, such as the 800XL.

If you have any of the new XL series computers, you have all six-
teen modes. If you have one of the older Atari 400 or 800 computers,
you lack some of the modes between 9 and 15. All of the old com-
puters have modes through 8, and some of the later ones have 9,

MODE

TYPE OF

NUMBER

MEMORY

NUMBER

MODE

RESOLUTION

OF COLORS

USED (BYTES)

Normal text

40 X 24

992

Large text

20 X 20

674

Large text

20 X 10

424

4-color graphics

40 X 20

434

2<olor graphics

80 X 40

694

4<olor graphics

80 X 40

1174

2-color graphics

160 X 80

2174

4<olor graphics

160 X 80

4190

High-res. graphics

320 X 160

8112

CTIA graphics

80 X 192

8138

CTIA graphics

80 X 192

8138

CTIA graphics

80 X 192

8138

4-color text

40 X 20

1154

4<olor text

40 X 10

664

2-color graphics

160 X 160

4270

4-color graphics

160 X 160

8112

Figure 8.4: The 16 graphics modes.

132 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

10, and 11 as well. Don't worry: even if you lack a few specialized
modes, you've got aU of the essentials. Just skip the descriptions of die
modes you don't have.

Widi diis said, we can proceed widi our tour of Atari graphics. In
these few pages, unfortunately, I can only give a brief overview. If
you want to know more, I'd suggest you read BiU Carris's book Inside
ATARI BASIC (Reston, 1983), or Atari Sound and Graphics, by Moore,
Lower, and Albrecht (Wiley, 1982). These go into much greater detail
than I can in this book.

Four-color Graphics S4odes. You have already been using one of
these: graphics mode 7. The other diree work in exacdy die same
way, but diey offer different degrees of resolution. Modes 3 and 5
have lower resolution, so dieir rows and columns are larger and their
coordinates smadler.

Mode 15 has even finer resolution than mode 7, with boxes only
half as high. Unfortunately, it also takes up twice as much memory—
ahnost die entire space available on the Atari 600XL. If you have an
Atari 800XL or a memory expansion, you will find mode 15 very
useful.

The paintbrushes and the SETCOLOR registers all work in
exacdy die same way in diese four modes. You can change a program
from one mode to die other merely by changing your coordinates to
reflect the different scale.

Two-color Graphics Modes. Modes 4, 6, and 14 are identical to the
four-color modes 5, 7, and 15, except diat diey lack two of the four
paintbrushes. You can still use paintbrush number 1, which starts out
orange, and die erasebrush (COLOR 0); however, you cannot use
paintbrushes 2 and 3.

What's die point of die two-color modes if diey don't do anything
different? They use only half as much space in die computer's mem-
ory, which can be important if you are writing a long program on an
Atari 600XL. If you only want to paint widi one color, you can save
space by usuig these modes.

High-resolution Graphics. Your Atari computer also offers you a
special high-resolution graphics mode. This is number 8, and it is
quite different from the others.

ADVANCED GRAPHICS 133

GRAPHICS 8 gives you twice the resolution of even the finest two-
and four-color modes — the points it plots are the smallest that can be
displayed on a normad television screen. This means that the com-
puter can draw diagonal lines that look almost exactly straight, with-
out the "staircase eflfect" you often see in other modes. You can use
this higher resolution to draw intricate pictures with fine detaiil.

You pay a price, though. To achieve this high resolution, you have
to sacrifice color: you can only draw lines of the same hue as the
screen background. In general, this meeins you will need to restrict
yourself to white.

The high-resdution graphics also take up a great deal of spjice in
the computer's memory. To use this mode on the Atari 600XL, you
must either write very simple programs or buy a memory expansion.

You do have some control over screen colors in the high-resolution
graphics mode, but the system for changing colors is not the same as
in the other graphics modes. Instead, it resembles the standard text
mode. SETCOLOR 2 chooses the background color, SETCOLOR 1
chooses the brightness of the plotted points and lines, and SETCO-
LOR 4 controls the border. As in the text mode, you cannot make the
points a different color from the background; you can only control
their brighmess. The SETCOLOR commands aflFect the text window
as weU as the graphics screen.

You really have only one graphics paintbrush in the high-resolution
mode, but you can turn it on and off with the COLOR commzmd.
COLOR 1 sets the paintbrush so that it draws lines, while COLOR
makes it erase by drawing with the background color.

If you want to use high-resolution graphics, I'd suggest you start off
with the foUowing sequence of commands:

GRAPHICS 8
SETCOLOR 2,0,0
COLOR 1

This selects the mode and changes the backgrcwmd color to black, to
make the graphics display stand out. You can certainly use other screen
colors, but this combination of white-on-black generally works best.

Special Graphics Mwles (CTIA). The XL series and some of the
more recent Atari 400 and 800 models have three other special
graphics modes, numbered 9, 10, and 11. These are sometimes called

134

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the GTIA modes, after the name of the electronic chip that Atari
installed to produce them.

These three modes are useful because they oflFer a wider range of
colors. In one of them, 11, you can have sixteen different hues on the
screen at once, which allows you to produce rainbow-like displays.
In another, mode 9, you can display 16 different shades of brightness
all at once.

The GTIA modes plot points that are slighdy wider than the nor-
mal graphics modes. While this is not a serious restriction on resolu-
tion, it makes it difficult to draw thin diagonal lines realistically These
modes are therefore better for pictures diat fill whole regions of the
screen with color.

The GTIA modes are unfortunately radier hard to use, because
they work quite differendy from the others. The wider range of color
choices makes the SETCOLOR statement more complicated, and
gives strange meanings to die paintbrush numbers in the COLOR
statement. Also, since these three modes do not preserve a text win-
dow at the bottom of the screen, diey can be used only in programs.

Mode number 1 1 is perhaps die most impressive of the three. All of
the points you plot must be of die same limiinance, but you can use
all 16 of the hues at once. You set die luminance of all the points by
putting a value into SETCOLOR register 4. You can dien choose
from 16 different paintbrushes widi the COLOR statement. The
paintbrushes are numbered dirough 15, to match die color codes in
the middle column of Figure 8.1.

Mode number 9 is a one-color version of die same idea. In this
mode, all of your points will be die same color, but you can use all 16
luminance values. You can choose die color of die points by putting a
value into SETCOLOR register 4, or you can use die default color,
gray The number in die COLOR statement chooses die brightness,'
not die color, in diis mode, and your 16 paintbrushes draw widi die
various luminances you select. This mode is particularly interesting,
because it allows you to use all 16 brightness values, not just die even-
numbered ones.

Mode number 10 is perhaps die most flexible, but unfortunately it
is also die most complicated. This mode allows you to use 9 different
paintbrushes at once, and lets you choose dieir hues and luminances
in any way you please. To use diis effectively, however, you wiU need
advanced programming techniques diat are beyond die scope of diis

ADVANCED GRAPHICS 135

book. Read Bill Carris's Inside ATARI BASIC if you are interested.

To get a feel for what these special modes can do, type the following
program:

10 PRINT "WHICH MODE";:INPUT MODE

20 GRAPHICS MODE

30 FORY = 0TO79

40 COLOR Y

50 PLOTO.Y

60 DRAWTOYY

70 NEXTY

80 GOTO 80

This is a variation on the solid-triangle program at the end of Chapter
7. The difference is that here the computer draws each horizontal line
with a different color.

When you RUN the program, the computer will ask:

WHICH MODE?

You czin choose amy graphics mode you want, although some of the
low-resolution modes will give you an error message, which you may
ignore. Press BREAK to stop the program when you're finished.
Then you can RUN it again choosing another mode. Mode 1 1 yields
a beautiful 16-color rainbow pattern. Mode 9 gives five gray bars,
which go through zdl of the gray tones from black to white (this is the
pattern shown in Figure 8.5). Mode 10 gives a series of colorful
bands, even though this program does not use dl of its 9 colors.
Modes 6, 7, 8, 14, and 15 also work with this program.

THE TEXT MODES

In addition to the 11 graphics modes, your Atari computer also has
five text mocks. You are familiar with one of them, the standard white-on-
blue letters which you have been using all along. This is GRAPHICS
mode 0, and it is by far the eaisiest to use. The others, however, do
have some usefiil features, such as control over the color of the letters.

The other four text modes resemble the graphics modes in some
ways, even though you cannot plot points or draw lines with them.
When you select them widi a GRAPHICS statement, the computer
gives you a black graphics screen. You can display letters in different
colors against this background.

136

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You use a vairiation on the standard PRINT statement to display
text in these special modes. Immediately after the word PRINT, type
#6 £ind a semicolon (;). Then type your message, enclosed in quota-
tion marks. A typical statement in one of these modes might look
like this:

PRINT #6;"HELL0"

The #6 lets the computer know that you want it to display the mes-
sage on the graphics screen; otherwise, it would show it as normal
white letters down in the blue text window.

Large Text. Modes 1 and 2 let you display larger letters than with the
normed text mode. They allow you to use only capital letters, but they
let you choose the colors. These modes are therefore useftil for large,
importJint messages, and for displays where you want multiple colors.
Because the letters are wider than normal text, you can only put 20
characters on each line. These two modes are identical, except that
the letters in mode 2 are twice as high as in mode 1 .

Figure 8.5: The special mode number 9 offers full control over shading.

ADVANCED GRAPHICS

137

The CAPS and inverse-video keys have a diflferent function in these
modes. If you PRINT lowercase or inverse-video messages, the
results are somewhat surprising: they will come out as capital letters,
but will have different colors. To see how this works, RUN the pro-
gram shown in Figure 8.6, taking special care to type the lowercase
Eind inverse- video messages as they are shown.

Figure 8.7 shows how you can use the SETCOLOR registers to
change the colors of each of the four kinds of letters you may type in
modes 1 and 2. Note that some of these color settings also aflfect the
color of the text window at the bottom of the screen.

Four-color text. Modes 12 and 13 are more specialized and harder
to use; however, they oflFer an interesting alternative. In these modes,
which are avaulable only on the new XL series, each letter is made up
of dots of four different colors. By choosing your colors carefully, you
can produce an interesting three-dimensional effect. Once again, the
two modes are identical, except for the height of the letters.

Figure 8.6: A program to display block letters in four colors.

138

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

To test out this mode, change the first line of the program in Figure
8.6 to read

10 GRAPHICS 13

Then RUN the program. At first you might not even recognize the
word HELLO, which the computer prints four times at the top of
the screen. This is because the preset color scheme tends to fill in the
middles of the letters. To make the screen more readable, type the
following commands in the text window:

SETCOLOR 0.2,2
SETCOLOR 2,9.12
SETCOLOR 3,4,10

You will now be able to read the message, and see that the parts of
each letter are made up of different colors.

This mode does display both uppercase and lowercase letters.

LETTERS
CONTROLLED

PRESET
COLOR

SETCOLOR
CODE

1
2
3

-. 4

Normal capitals
Normal lowercase
Inverse capitals
Inverse lowercase
Background

Orange

Green

Blue

Pink or Red

Black

0,2,8

1,12,10

2,9,4

3,4,6

4,0,0

ffgure 8.7: How to control the colors m the text modes 7 and 2.

PART OF

SETCOLOR

LETTER

PRESET

SETCOLOR

CONTROLLED

COLOR

CODE

Left side

Orange

0,2,8

Middle

Green

1,12,10

Right side of
normal letters

Blue

2,9,4

Right side of
inverse letters

Pink or Red

3,4,6

Background

Black

4,0,0

figure 8.8: The SETCOLOR regiiters in the four<oior text modes 12 and ?3,

ADVANCED GRAPHICS 139

Inverse video, however, controls the color scheme, as you can see
from the third and fourth copies of the word HELLO on the screen.
In these, the blue part of each regular letter is shifted to red, while the
other colors remain the same. Figure 8.8 shows the meanings of the
SETCOLOR registers in these two modes.

With the special text modes, we have reached the end of our tour of
the Atari's 16 graphics modes. This has been just a brief introduction
to one of the most attractive features of your computer, and I hope
you will continue to explore it.

SUMMARY

The Atari computer has an attractive and flexible graphics system.
Using the advanced techniques in this chapter, you can tap the poten-
tial of this system.

In graphics mode 7, the SETCOLOR command allows you to
chamge the preset colors of your padntbrushes. You can choose ziny
combination of 16 colors and 8 luminance values for the points and
lines you draw on the screen. In the text modes, the SETCOLOR
command controls the colors of the letters, background, and border.

By adding 16 to the number of any graphics mode, you can elimi-
nate the blue text window and extend the graphic display all the way
to the bottom of the screen. You can only do this in a program.

Of the 16 Atari graphics modes, 11 allow you to draw points and
lines for a true graphic display. The 1 1 modes differ in the number of
colors and the amount of resolution they oflFer. If you mzister the spe-
cial GTIA modes, you can use many diflFerent colors on the screen at
once to produce beautiftU rainbow eflFects.

The other five modes are for text display. By using the large text
and four-color text modes, you can display messages in oversize let-
ters and in color.

You have now reached the end of Section Two of this book. You
know how to write progrEims, use variables, and control the order in
which the computer follows your commands. You have had a glimpse
of the potential of your graphics system, and are now ready to explore
your computer on your own. The ftm is just beginning.

Section

Storage

B CHAPTER NINE

Cassette
Storage

In the first two sections of this book, you have been concentrating
on the Atari computer itself. Everything presented up till now could
be accomplished with nothing more than your computer and a televi-
sion set; no additional equipment was required.

In this third section, you will learn some of the things you can do if
you have additionzd equipment to connect to your computer. This
chapter focuses on the Atari Program Recorder, which uses a cfissette
tape to store programs and data. Chapter 10 considers the disk drive,
which has a similjir function. It is more expensive, but is faster and
more useful. Chapter 11 will discuss some other equipment you can
add to make your system even more versatile.

There aie two main reasons why you might want to add a program
recorder or a disk drive to your system. First, many of the most inter-
esting programs for the Atari are sold on cassette tape or on diskettes.
If you want to use these, you will need a device that can load the pro-
gram into the computer's memory.

The second reason is permanent storage. When you type a pro-
gram into your computer's memory, it remains stored until you give a
NEW commzmd or turn off the power. That is fine if you just want to
use the program for a few minutes, then go on to something else.
Many people, however, like to write programs on their computer,
then store them for future use. With a cassette recorder or disk drive,
you cjin save the progreim, turn the computer off, and load the pro-
gram back in when you want to use it again. In this way, you can
write a complicated program and have it permanently aveiilable.

In this chapter, you will leaim how to set up your Atari Program

144 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

Recorder and how to use it. You will find out how to load commerical
software from cassette tapes into your computer, and how to store
your own programs on tape.

SETTING UP

THE PROGRAM RECORDER

There are two versions of the Atari Program Recorder. The one
currendy being sold is the model lOlO.This compact model works
well and matches the styling of the GOOXL and 800XL computers.

If, however, you bought your system some time ago, you may have
purchased the earlier version, model 410. This older version works
exacdy the same as the 1010, so you can use either model with any
Atari computer.

To set up either type of program recorder, you need to make two
connections. First, you must attach the thick black cable from the
back of the recorder to the wide hole labeled PERIPHERAL on
the back of the computer. The plug will only go into the hole one way.
If it doesn't seem to fit, try turning it over. The second connection is
easy: just plug the recorder into a power oudet.

The program recorder is identical to a normjd cassette player,
except that its speciad connection lets the computer turn it on and off
automatically The RECORD, PLAY, REWIND, and ADVANCE
buttons work exacdy as they do on a regular c£issette deck, and you
can use normal cassette tapes in it. Unfortunately, you cannot use a
normal cassette recorder for this purpose: you must have the Atari
recorder with the special cable.

If you are using prerecorded pn^;rams, you usually won't need to buy
blank tapes of your own. Normally, aU you'll need to do is load the pro-
gram fix)m the prerecorded cassette tape into the computer's memory.

If you want to store your own programs or want to use a special
program that does expect you to store information on tape, you will
need to buy some blank cassettes. You can use any normal cassette
with your program recorder. Short tapes such as C-30s are usually
better, since longer ones can stretch and become unreadable. You
don't need to buy special high-fidelity or computer tapes. The cheap-
est audio tapes work just as well.

Take care of your tapes/ Since your tapes may hold an expensive pro-
gram or valuable data, you will want to be careful in handling them.

CASSETTE STORAGE 145

You must load your program from the tape every time you want to
use it, and even a minor flaw can make it unreadable.

Always rewind your tapes to the beginning before you remove
them from the recorder. Most tapes have a blank plastic leader at the
start of each side, so that the recorded portion will not be exposed. If
you don't rewind to this leader, you may dzimage the tape surface as
you remove it.

Keep your tapes away from all sources of magnetism, which can
erase the recorded information. Even your television and telephone
can produce enough of a magnetic field to daimage a recording. Keep
your progrEun recorder at least two feet away from your television,
and never scatter taf)es carelessly around your work area.

LOADING PRERECORDED

CASSETTE PROGRAMS

Many of the programs commercially available for the Atari com-
puters are sold on cassette tape, because czissettes are easy to use and
widely available. When you buy one of these progrzims, you get a
single cassette with the program recorded on it. To use the program,
you need to load it into the computer's memory.

Since prerecorded cassette programs are made by many diflferent
companies, it is impossible to give general loading instructions that will
work for all tapes. You should read the instruction sheet that comes
with the package to find out the specific commands for loading that par-
ticulzir program.

While I cannot give you a procedure that will work for all cassette
programs, I can tell you the most common one. Do the following
steps in order:

1 . Type:

CLOAD

2. Press the RETURN key. You will hear one beep.

3. Put the tape in the program recorder and rewind it to the
beginning of the side on which the program is stored.

4. Press PLAY on the program recorder. The tape will not
start moving yet.

146 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

5. Press the RETURN key again. The tape will start to
move and you will hear a beeping sound through your
television.

6. Wait until the tape stops turning in the recorder. If the
computer says READY, your tape has loaded correctiy.

7. Press STOP on your program recorder and rewind the
tape. The program is now in the computer's memory, and
you can RUN it like any other prograun.

While the tape is loading, you cannot type other commands on the
keyboard. If, at any point, you want to stop the procedure without
loading the prograim, you can press the BREAK key. The computer
will say READY, 2ind you can type commamds again.

If you get an ERROR, your computer had problems loading the
tape. Rewind it and repeat this CLOAD procedure. If you still get an
ERROR, you may be trying to load a tape that was designed to be
loaded in some other way — check the instruction sheet that came with
the program.

Unfortxmately, you also might have a tape that is defective or that
has become unreadable with use. Cassette tapes are very delicate, and
can be easily daimaged if you don't take care of them. If you cannot
loaid a tape that you have loaded successfiilly in the past, it probably
means that the coding has become unusable.

You can try any of three things. First, turn the tape over to the
opposite side, rewind it, and repeat the procedure. Most software
manufacturers duplicate the program on side two of the tape, just in
case you have problems with the first copy.

If that doesn't work, you can try to RUN the program anyway, in
spite of the loading error. While this rarely works, it is worth a try.

As a third and final step, check with the dealer who sold you the
program. Because cassettes frequently have problems, mziny manu-
facturers will exchange a cassette that has become unreadable.

SAVING YOUR OWN PROGRAMS

If you want to store one of your own programs on a cassette, you
can use a procedure called CSAVE, which is very similar to the
CLOAD command. Type your program into the computer's memory.

CASSETTE STORAGE 147

You don't need to worry about length: you can store hundreds of pro-
gram lines on even a short cassette. When you have your program the
way you want it, type:

CSAVE

The computer will beep twice. This is your cue to put a cassette in the
recorder, rewind it, and press the RECORD and PLAY buttons. As
on a normzJ cassette recorder, you must press both buttons at the
same time to record on the tape.

From here on, you follow exactly the same procedure as you did
to load a program with CLOAD (steps 3 through 7 on pages 145-46).
You press RETURN to stau-t the tape, then wait until the recorder
stops and the computer says READY. The program is now stored on
the tape.

If this is a very important program, or a long one that would take
hours to type back in, this tape is very valuable. Rewind it and mark
it carefully. Put it in a plastic case if you have one, and store it some-
place where it is safe from accidental erasure.

You can also protect your tape from accidents by punching out one
of the small plastic tabs along the top of the cassette. As with an audio
cassette, the Atari prograim recorder will not record on a tape that has
this tab punched out.

To protect your cassette in this way, hold it with the recorded side
facing you. Then use a pen to punch out the tab near the upper-left
comer, as shown in Figure 9.1. The other tab protects the second side
of the tape.

If you later decide you do want to record over a protected cassette,
you can still do so. Cover up the hole with a piece of cellophane tape,
then record your program, just as you normally would. The recorder
will think the plastic tab has not been punched out, and will let you
record over the tape. Before you press the RECORD button, make
absolutely certain that this is what you want to do.

If you want to be very sure that you don't lose a program, you
might want to keep it on zuiother tape as a backup. Put a different
blank cjissette in the drive and type another save command. When
the computer has finished saving the program you will have your
backup copy on another cassette. Keep the backup in a different
place. That way, if zinything ever happens to your first tape, you can
still load the program from the second.

148

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You lo£id a stored program back into the computer with a CLOAD
command, just as you would a prerecorded cassette program. Follow
the CLOAD procedure oudined on the previous page, then LIST the
program to make sure it's the one you want.

Whenever you give a CLOAD command, the computer clears any
program you may have stored in its memory, so that leftover lines
won't interfere with the new program. This will erase your old pro-
gram just as a NEW command would. If you have been working on
an important program, you should save it before you load another in.

Since the CLOAD and CSAVE procedures are so simUar, it is easy
to forget which you are using. If you mistaikenly press the RECORD
button when you were merely trying to load a prograim back into the
computer, you may erase a valuable program.

One way to remember is to notice the number of beeps. When the
computer wants you to press only the PLAY button, it will beep once.
When it wants you to press both PLAY and RECORD, it will beep
twice. Just remember: one beep, one button; two beeps, two buttons.

Figure 9.1; Protect your tapes from accidental erasure.

CASSETTE STORAGE 149

It is possible to record more than one program on each side of the
cassette tape, but you have to be very careful. You can keep track of
several programs on a single tape by watching the small mecheinical
counter on top of the program recorder. If you feel certain that you
won't record one on top of another, you can try to squeeze several
on a side.

When you load a tape on which you want to record multiple pro-
grams, press the small reset button next to the counter on the
program recorder, so that the counter reads 000. Then fast-forward
the tape to well beyond the end of the first progrjim. Note down the
counter reading, then record your second program. When you want
to load this second program back into the computer, you can return to
its place on the tape using the counter.

Use this technique only if you're recording large numbers of pro-
grzims and don't care if you lose them. You can conserve tapes by
recording programs together, but you c£in easily erase one by accident
if you're not careful. Unless you really know what you're doing, stick
to one program on each side of a tape.

OTHER WAYS TO SAVE PROGRAMS

The CSAVE and CLOAD commands are the simplest way to save
or load a program to or from a cassette tape. There are, however, two
other pairs of commands that work in much the same way, but give
slightly different results. These other commands are also more general,
because they will also work for storing programs on other devices such
as the disk drive.

With all of these commands, you will be using the basic seven-step
procedure for controlling the program recorder. You first type your
command, then press the RETURN key. The computer will beep
once or twice, depending on whether it wzints you to press just PLAY,
or to press PLAY and RECORD together. You then set up your tape
in the recorder, and press the RETURN key to set it in motion. The
computer will stop the tape and say READY when it has finished sav-
ing or loading the program.

The first pziir of commands is SAVE and LOAD. These do exactly
the same thing as CSAVE and CLOAD, except that they are more

150 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

general commands that work with other devices as well. These com-
mainds look like this:

SAVE "C:

and

LOAD "C:"

The "C:" tells the computer that you want to use the cassette
recorder to save or load the program. This letter C, which must
always be followed by a colon, is the standard name for the program
recorder, and you must specify it in all cassette commands except
CSAVE and CLOAD.

The SAVE and CSAVE commands use diflferent coding formats to
store the program on the tape. You cannot use LOAD on a program
that was saved with CSAVE, or vice versa. As with CLOAD, the
LOAD command erases any program that you may have stored in
the computer's memory.

Why use these expanded commands? For one thing, the SAVE for-
mat lets you load cind run a cassette program in a single step:

RUN "C:"

This tells the computer to load the cassette program into its memory
and RUN it immediately With a CSAVE tape, you would have
needed to give two commands:

CLOAD
RUN

LIST and ENTER are the other pair of commands. This LIST
statement is a variation of the command you have used since Chapter
5 to display all or part of a program on the screen. To store program
lines on a cassette with this command, type:

LIST "C:"

The "C:" tells the computer to save the program lines on the cassette
tape, rather than displaying them on the screen. You can then reload
them by typing:

ENTER "C:"

Once again, you can only use ENTER with a program that was
saved with a LIST statement. Programs stored with CSAVE or SAVE
"C:" must be loaded with their own commands.

CASSETTE STORAGE

The great advantage of LIST and ENTER is that they let you set
aside parts of a program. If you only Wcint to save the program lines
from 40 through 70, you can type the command:

LIST "C:",40,70

The computer will record lines 40 and 70 and those with numbers in
between. If you later use ENTER to load the program back in, these
are the only lines that will be retadned.

This is very useful if you wzint to save one useful subprogrzim out
of an entire program. You can save the subprogram with this LIST
command and later ENTER it into other programs.

Unlike CLOAD and LOAD, the ENTER command does not clear
the progreim memory before it loads the new program. It merely adds
the lines to the program that is already stored. If you do want to clezu'
the memory, type a NEW commaind before you type ENTER.

You can even use this command if you need to delete a large num-
ber of lines from a program. Normally, you would have to type each
of the statement numbers one by one for each of the lines that you
want to delete. With a LIST "C:" command, you can save the
desired part of the program on tape, clear the memory with a NEW
command, and reload the part you saved with ENTER "C:".

SUMMARY

With an Atari Program Recorder, you can run commercial pro-
grzmis that are sold on cassette tapes, and you can save your own
programs, so that you won't have to type them in each time you waint
to use them. This makes it practical to run much longer prograims.

CSAVE and CLOAD are the two most important commands for
saving and loading programs on cassette tapes. When you use these,
the computer asks you to position the tape and press the appropriate
buttons on the program recorder. The computer then turns the
recorder on and ofiF, as it needs to save or read information.

SAVE "C:" and LOAD "C:" are alternative commands that you
can use. These are more general than CSAVE and CLOAD, since
they can also be used with other devices, such as the disk drive. You
can load and run programs stored in this form with a single com-
mand, RUN "C:".

151

152 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You can also use LIST "C:" and ENTER "C:" to store a pro-
gram. These work somewhat differently, since they let you store the
program line by line. These commands are usefiil when you want to
save only part of a program.

B CHAPTER TEN

The Disk Drive

With an Atari Disk Drive, you can store your information in more
useful ways than with a cassette recorder. You can use prepro-
grammed software almost instandy, without having to wait for the
program recorder to read through the tape. A disk drive lets you save
many different programs on the same diskette easily and reliably.

A disk drive is a mechanical device that can read and record infor-
mation on a flat magnetic surface. It acts somewhat like a record
player, spinning a circular diskette at high speed and recording infor-
mation in thin bands on its surface. Unlike a phonograph record,
however, the diskette can have its information changed or replaced. In
this way, the disk drive acts more like a tape recorder

A diskette, shown in Figure 10.1, is a 5V4-inch vinyl plate, covered
with a magnetic recording surface. To protect it from damage, the
magnetic disk is encased in a plastic envelope, with only a few small
holes exposed. In operation, the drive spins the disk inside its enve-
lope and records information on the surface as it passes.

Because they were designed specifically for computer data storage,
diskettes are far superior to cassettes. Diskettes are made to spin much
faster, so the computer can save or retrieve much more information in
a given time. The information is recorded more efl&ciendy on the flat
surface, so a diskette cjin hold more than a tape.

The greatest advantage of disk storage is its flexibility. With a tape,
you can only start at the beginning and read straight through to the
end. With a diskette, however, your computer can go immediately to
any spot you want and find the information that you want to retrieve.
A diskette, therefore, lets you save many different programs, then
immediately find the one you are looking for This flexibility allows

154

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

you to use disk drives to do many things that would be impractical
with a program recorder.

You can connect as many as four disk drives to your Atari com-
puter, but one is enough for most people. For most of this chapter, I
assume that you have connected only one drive to your computer.
Multiple drives are discussed briefly at the end.

SETTING UP THE DISK DRIVE

Atari has sold two diflferent versions of its disk drive. The original
drive was the model 810, shown at the right of Figure 10.2. The

Figure 10.1: A typical diskette.

THE DISK DRIVE

155

newer model 1050, shown at the left of the figure, is an improved ver-
sion, which works faster and can store more information on each disk-
ette. The dififerences, however, are fairly minor, and both models
work with Jill AtJiri computers.

Because it is a more sophisticated piece of equipment than the pro-
gram recorder, the disk drive is slighdy more complicated to set up.
The basics are the same, however, and you can lezim the rest easily.

Before you start, make sure the power switches on the computer
and the disk drive are both turned off. Never connect a disk drive to
the computer with the power on.

Start by connecting the large black cable from the disk drive to the
the computer. This cable is just like the cord that cormects the pro-
gram recorder, with a wide black cormector on both ends. Plug one
end into one of the large I/O CONNECTOR holes on die back of the
disk drive: either one will do. Then cormect the other end to the hole
marked PERIPHERAL on the back of the computer.

If you want to use a program recorder or another peripheral in
addition to the disk drive, you can plug its cable into the other I/O
CONNECTOR hole in the back of the drive. By chaining devices
together in this way, you can connect as many as you want. The pro-
gram recorder must always be at the end of the chain, because it does
not accommodate two cormectors.

Look in the small hole on the back of the disk drive marked
DRIVE SELECT or DRIVE CODE. In this hole, diere is a pair of

Figure T0.2; The old and new Atari disk drives.

156

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

smaU switches that tells the computer which disk drive this is. The
second switch may be hard to see at first, but it is direcdy behind die
first one and butted up against it. If you are using only one drive,
both of diese switches should be pushed to the left to choose drive
number 1, as in Figure 10.3. To the computer, this drive will be
known as Dl .

The disk drive has its own power supply, which should have come
packed widi the unit when you bought it. Connect the smaU end of its
cable to the PWR hole on the back of the drive, then connect the
power cord to a waU outlet. That's it for the connections.

TAKING CARE OF DISKETTES

Diskettes are fragUe objects. If you treat them with care, they will be
reUable, but if you abuse them, you can damage diem permanendy.

Don't bend your diskettes. While you can flex them slighdy widiout
hurting them, you will make them unreadable if you fold them
beyond dieir limits. Be very careful not to bend them as you handle
them or insert them into the drive.

Keep dirt and dust away from your diskettes, and never touch any
of die exposed portions of the recording surface. The magnetic coat-
ing on die diskette can easily be damaged by any contamination. Be

Figure 10.3: Set both switches on your drive to the left, unless you are usine

more than one drive

THE DISK DRIVE 157

especially careful of the oblong hole that extends outwards from the
center. This is where the hejids of the disk drive make contact with the
diskette to store and retrieve information.

Always slip your diskette into its paper sleeve when you aren't
using it in the machine. Store diskettes at room temperature, in a
place where they won't be disturbed. Some computer stores sell plas-
tic cases that protect diskettes in storage.

When using a diskette in a disk drive, always follow one cardinaJ
rule: Never insert or remove a diskette while the drive is turning. Always wait
until the drive stops and the red "busy light" goes out. Also, remove your
diskettes before you turn the drive or the computer off. If you should ne^ect these
rules, your disk drive mi^t erase some of the information stored at the
place where it was touching the diskette, and might even damage the
recording surface.

There is one bit of protection built into the diskette system: the
small write-protect notch along the side of the envelope. Normally, you
can both read and record information on the diskette. However, if
you cover up this notch, as in Figure 10.4, you protect the diskette
from recording. The disk drive can still read information from the
diskette, but it cannot record over or erase anything. If you later
decide you do want to record on the disk, you can easily remove the
covering from the notch. Most commercial programs are sold with the
notches covered or absent altogether, so that you cannot erase any-
thing. Of course, covering the notch will not protect your disks from
other types of damage.

I hope you won't feel intimidated by all these "Don'ts." In my expe-
rience, diskettes are surprisingly resilient, and they will often survive
abuse that you might expect would destroy them. Still, considering the
amount of valuable information you may have stored, it is generally
best to play it safe: if you treat your diskettes ri^t, they will give you
fsdthftil service.

TURNING ON THE DISK DRIVE

No matter how you plan to use your disk drive, you must start it
up with a prerecorded diskette. This can be either Atari's Disk Operat-
ing System (DOS) diskette that came with the disk drive, or a commer-
cial softwzire package designed directly for use on the disk drive. If
this is your first time, I'd suggest you use the Atari DOS diskette.

158

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

On the front of the drive, there is a small slot or door, where you
will insert your diskettes. The door looks a litde different, depending
on whether you have an old or a new model, but the general principle
is the same.

On the model 1050, you will slide your diskettes into a very diin
slot. Toward the left side, there is a small lever that you can pull
down. When you do this, you are clamping the drive mechanism onto
the diskette, so that it can spin and record data. When you want to
remove the diskette, wait until the "busy light" goes off and then lift
the lever. You can dien puU die diskette out.

On the older model 810, you slide your diskette into a fairiy wide
slot. To lock the diskette into place, you pull the black plastic door
down until It clicks. To open the door and remove the diskette, press
the button just under the door, to the left of the words ATARI 810

Whichever model you have, you use the same procedure for

Figure 10.4: By covering the write-protea notch, you can keep the disk drive
from erasing information.

THE DISK DRIVE

159

turning on the disk drive and computer. Make sure you follow this set
of directions in this order every time you use the disk drive with your
computer:

1. Before you turn the drive on, make sure that the door is
open and there is no diskette inside. You can ruin a disk-
ette if you turn the disk drive on or off with it inside. Your
computer should also be off at this point.

2. Turn on the empty drive. The red PWR ON light and the
"busy light" should go on and the drive should whirr into
action. Don't worry if it sounds like it is trying to shred a
piece of paper: that is its normal noise. After a few sec-
onds, the "busy light" will go out and the drive will stop
with a crunch. Don't do anything until the drive stops.

3. Once the drive is quiet and the "busy light" has gone out,
insert the diskette you want to use. This must be either a
prerecorded software package or the Atari DOS diskette —
you cannot use a blank diskette yet. To insert the diskette,
slide it into the slot on the drive with the label facing
upward and the oval hole towards the back of the
machine. Be extremely careful about which way you
insert the diskette. Since it is square, it will fit in the slot
in many ways, but it will only work when you use the
proper orientation, as in Figure 10.5.

4. Push the diskette all the way into the slot, until you feel it
click into place. On the 1050, there is a slight depression
to the right of center in the slot, which allows you to push
the disk the last eighth of an inch in. Close the door on
the drive. The drive mechanism will spin for a few sec-
onds to center the disk, then stop. It now holds the disk-
ette mechanically, and can read information from it.

5. Turn on your computer and your television. The drive
will start to whirr again, for ten to twenty seconds. While
it is working, the computer shows only a blank blue
screen. When the disk drive stops, the computer finally
says READY, to show that you can type commands as
always. You are ready to roll.

160

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

That is the ideal case. It can happen that your drive will keep turn-
ing and your computer will display the message

BOOT ERROR

This indicates that you have done something wrong as you were fol-
lowmg this procedure. It generally means either that you were using a
diskette that your computer could not read, or that you did not insert
It correctly Check yourself against Figure 10.4.

If this happens to you, the drive will just continue to whirr, and you
wiU get a long string of BOOT ERRORs. Even though the red light
does not go out, you will need to open the door on the drive to release
the diskette and slide it out of the drive. Once you have done this,
you can turn the power off on your computer and wait for the drive to
crunch to a halt. Then turn off the disk drive and start over.

Why do you have to have a disk in the drive when you turn the
computer on? When you use your Atari computer with a disk drive

Figure 10.5: How to insert a diskette.

THE DISK DRIVE 161

it must load some additional instructions into its memory so that it
will know how to respond to disk commands. The Atari computer is
designed to look for these additional instructions on the diskette you
are using, then load them automatically into its memory as it is start-
ing up. This is why you must use die Atari DOS or another prere-
corded diskette: if the computer cannot find the instructions on the
diskette, it will not be able to turn itself on properly.

USING PRERECORDED
DISKETTE PROGRAMS -

As with prerecorded cassettes, there is no way to generalize about
loading commercial diskette software. The best thing you can do is
read the instruction sheet that came with the program: it will surely
contain detzdled information on how to load the program.

Some diskette programs load and start themselves direcdy when
you turn on the computer with the diskette in the disk drive. Others
start the computer up and display the word READY. In this case, you
will usually have to type a command such as

RUN "D:PROGRAM"

where PROGRAM is the name of the package you are using. Check
the instruction sheet for details.

When using a commercial diskette program, you should be espe-
ci£illy careful to follow the five-step startup procedure precisely. Disk-
ette programs can be quite expensive, and a single mistake can make
them unreadable. Most software manufacturers will replace a dam-
aged diskette for a small fee, but they make you go through a lot of
trouble to do this.

In some cases, you can make a copy of the program on a blank
diskette, using the instructions on pages 168-69 later in this chapter.
This allows you to use the copy and keep the original safe in case you
have problems.

Many programs, however, are designed so that you c£innot copy
the diskette. Software companies do this to keep you from selling or
giving unauthorized copies to a friend: as with books, most computer
programs are copyrighted, and cannot be duplicated without permis-
sion from the author. W'tJi a program that has been protected in this
way, you must be very careful not to ruin the original diskette.

162

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

THE DOS DISK

We have already encountered the Atari DOS diskette in the proce-

MA^rip nJ^iS^.' "^"^ ^"'- ™^ ^'^^^«^' «°™^t™^« labeled
MASTER DISKETTE, contains the instructions that teU the com-
puter how to use the disk drive: if you lose this disk without making a
copy, you wiU have no way to use the computer with the drive DOS
stands for Disk Operating System, and is actually a program that
runs unseen m the computer's memory.

Atari has three different versions of its Disk Operating System-
DOS 1.0, DOS 2.0S, and DOS 3.0. DOS 1.0 Was the ongind ^S-
sion and was quickly replaced by DOS 2.0S. These two versions
work m almost exactly the same way, and can be used on either the
old or the new disk drive.

With the introduction of the new 1050 drive, Atari has announced
a new version of this DOS program. This new version, DOS 3 is
designed to take advantage of the 1050's faster speed and higher-
density storage, and it includes many other improvemems over the
earlier versions. It wiU work on the old disk drives as well.

Unfortunately, as this book is going to press, DOS 3.0 is still not
available and is not expected for some time. For this reason, all of the
descriptions in this book are directed toward DOS 2.0S, the most
common version currendy being sold. DOS 3.0 will work 'very "simi-
larly when It becomes avaUable, so you will not be missing anything
essential by reading about the older version.

When you turn on the computer with the DOS disk in the drive a
part of the DOS program is automatically loaded into the computer's
memory. This lets the computer respond to disk commands that it
would not othenvise recognize. You can use your computer just as you
normally would, but you can also use these new commands that have
been added to its vocabulary. These new commands include SAVE
LOAD, and RUN, which will be discussed later in this chapter

You can also use the DOS program in a more visible way bv
typmg the command

DOS

This switches the computer out of its habitual mode of accepting
BAMG commands and program lines, and reads in another part of
the DOS program. This is a set of disk utilities that lets you do special

THE DISK DRIVE

163

operations on your diskette. Note, incidentally, that when you use the
DOS command, any BASIC program you had stored will be lost.
Save it first.

Type the DOS commzind £ind press RETURN. The disk drive will
whirr again and the display will chainge to look like Figure 10.6. This
is caUed the DOS Mmu, because it lays out all of the selections you can
make. In most cases, you wiQ give your commands by typing one let-
ter out of this list, then pressing RETURN. If the computer needs
more information to carry out the operation, it will ask you questions
on the screen.

While you still have your DOS disk in the computer, type command
A; to ask for the DISK DIRECTORY. The directory is the hst of the
names of all the programs and data files stored on the disk. When you
press RETURN, the computer will respond with a cryptic question:

DIRECTORY -SEARCH SPEC.LIST FILE?

It is asking you whether you want to search for a specific name or list
all the files stored on the disk. Press RETURN to see the contents of

&I5.K OPERATING =• Y 5 T E M II UER'^IDM Z b'
COPYRIGHT i5«E.e ft T & P: I

DISK C' I P: E C T R: S'

I .

F D R M (J T C' I S K

RUM CCiRTRIC'GE

..1 .

^anam*«Jh*:^A*A'i

COPY FILE

h; ,

t-XHUW.W S. C U E

DELETE FILEtSO

L ,

B I H Ci R Y L D Ci D

RENCiME FILE

M .

RUM (JT CiC'C'RESS

LOCK FILE

N .

CRE6TE MEM . SCiU

LIMLDCK FILE

D .

CUPLICfJTE FILE

URITE COS FILE

ELECT ITEM OR:

F Ci R M E M U

figure 10.6: The DOS Menu.

164

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

the entire disk. If you have version 2.0S of the DOS program, the
computer will probably list three names, something like this:

DOS SYS 039

DUP SYS 042

AUTORUN SYS 001
625 FREE SECTORS

The names of the programs and the number of FREE SECTORS
may vary slightly with your diskette. This number shows how much
space you have left on the diskette (625 is a lot).

When you have many other files stored on the disk, the directory
might fill up die entire screen and even roU some of die names oflF die
top. To keep diis from happening, you can stop die movement of die
list by pressing CONTROL and die number 1. Proceed like diis:
give die directory command as usual, dien press CONTROL- 1 as die
listing appears on die screen. It wiU stop where it is, so diat you can
read it. When you're ready to go on, press CONTROL-1 again and
the listing will continue to the end.

Since you have only one DOS diskette, it is best to prepare a blank
diskette for use in its place. You can buy blank diskettes at any com-
puter store or Atari retailer. Atari seUs blank diskettes under its own
name, but you can also use any number of odier good brands. Disk-
ettes are usually sold m cartons of five or ten.

To use a blank diskette, you must &rst format it so diat die disk drive
can read it. When you ask die computer to do diis, it teUs die disk
drive to put magnetic markings on die diskette so diat it can find its
place as it moves around. You don't have to worry about how diis
works: just format die disk, and die computer will keep track of where
things are.

Remove the DOS diskette from die drive and insert your blank
diskette. Then type I, for FORMAT DISK. The computer wiU ask

WHICH DRIVE TO FORMAT?

Unless you have more dian one drive, you should answer 1 or Dl.
The computer wiU then ask you to

TYPE "Y" TO FORMAT DISK 1

It asks you this to make sure you really want to go ahead with it.
Make sure your diskette is actually blank: Formatting wiU erase any

THE DISK DRIVE

information that you had stored on the disk. If you're certain this is
all right, type Y and press RETURN. The drive will dick for about
45 seconds, then stop. You cannot give further commands until the
"busy light" has gone out.

Before you can use your blank diskette as a duplicate DOS disk,
you need to do one more thing. Type the H command, WRITE
DOS FILES. This tells the computer to take the DOS program stored
in its memory and copy it onto your blank diskette. Answer Dl and
Y to the same two questions, then wait while the computer shows the
message

WRITING NEW DOS FILES

When the drive stops, your blank diskette contains all of the informa-
tion the computer needs to control the drive. You can use this disk as
if it were the originad DOS disk.

I would suggest you go through this procedure every time you
begin using a new diskette. Fire up the disk drive using a diskette that
already contains the DOS program (this can be either the originad
disk or smother disk you have already prepared). Format the new
blank diskette and write the DOS files to it. You can then use this disk
as a "parent" for others, and keep the original DOS diskette safe.

165

FILES

The rest of the blank diskette is yours to play with. You can store
programs, data, or other information in any way you wish.

Your computer organizes the disk in Jiles. These are logical blocks of
information that you want to use all at once in the computer. A stored
program is normally a sin^e file — you save it as a unit, then reload it
all at once. Other common files include blocks of data (such as a mail-
ing list), and system utilities, such as the DOS program.

Every file has its own name. The computer automatically keeps
track of the place where esich file is recorded, so all you need to do is
refer to it by its name. The disk directory (selection A on the DOS
menu) is an index of the names of all the files on the diskette. You can
store up to 64 files on an Atari diskette.

The name of a file can be up to eight letters long. The file name
can contain numbers, though not as the first character. Spaces and

166

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

punctuation marks are not allowed. As with variables, it is best to
choose a name that explains the file's contents. Otherwise, you are
likely to forget where the information is stored.

You can add an optional three-letter extension following the file
name. Many people use this to explain what type of information is
stored (BAS means a BASIC program, DAT means a data file, SYS
stands for a system program, such as DOS). You can also use the
extension to distinguish diflferent versions of a program.

The fiJl name of a file cbnsists of three parts: the drive number (Dl ,
unless you're using more than one drive), the file name, and the exten-
sion. Always type a colon (:) between the drive number and the file
name, and a period (.) between the file name and the extension. You
can usuaUy leave oflf the drive number if you are just using a single
drive, and you can omit the extension if it is blank. The foUowing are
some valid file names, in their fiill forms:

D1 rDOS.SYS
D1:FILENAME.EXT
D1:PROG137.BAS
D1:NAME0NLY (no extension)

Now that you know how to give names to files, you can use some of
the other options on the DOS menu. With options C, D, and E, you
can copy, delete, or rename any file on the disk. As an example, let's
make a copy of die DOS. SYS file, and caU it NEWCOPY T)^e C
and press RETURN. The computer should respond

COPY- FROM, TO?

It is asking you to tell which file you want to copy, and what name
you want to give to the new file. Type this line exacdy:

DOS.SYS.NEWCOPY

Note that we omitted the drive number Dl from the file names (the
computer assumes this unless you tell it otherwise). The two file
names are separated by a comma in this line.

When you press RETURN, the disk drive will start to whirr, mak-
ing a second copy of all the information stored in the DOS. SYS file.
When it has finished, you can check to see diat the copy is also on the
disk, by asking for a disk directory (command A). If you are working

THE DISK DRIVE

with a newly-formatted diskette, the computer will respond something
like this:

DOS SYS 039

DUP SYS 042

NEWCOPY 039
587 FREE SECTORS

Try out the E-RENAME and D-DELETE commands on this
NEWCOPY file. They both work in simQar ways to the C-COPY
command, though with DELETE the computer stops to ask you

TYPE "Y" TO DELETE . . .
D1:NEWC0PY?

It does this to make sure you really do want to delete the file. Once
you type Y and press RETURN, the file is gone forever.

SAVING YOUR OWN PROGRAMS

167

If you have been writing your own programs, you can store them
conveniently as files on a diskette. You can save many different pro-
grams on a single diskette, then call them back by name as you need
them. You can build a library of all the programs you have written, so
that you never have to retype an old program. You simply find the file
on the diskette and run the stored program.

To type and save a program, you must leave the DOS menu and
return to the normal BASIC programming language. You do this by
typing the B command, shown on the DOS menu as RUN CAR-
TRIDGE. If you have one of the new XL machines with built-in
BASIC, you might find this phrase confusing: it is left over from the
time when Atari BASIC was a czirtridge that had to be plugged in.
With the new XL machines, this command simply sends the com-
puter back to its familiar READY prompt, to show that you can
agziin type normal commands and progrzim lines. Atari will probably
change the words RUN CARTRIDGE in a future revision of its
DOS program.

When you have the word READY on the screen, type in your pro-
gram, exacdy the way you want it. RUN it to make sure it works,
then use the following command to store it on the diskette:

SAVE "D:FILENAME.EXT"

168

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

FILENAME and EXT can be any name and extension that you want
to give to your stored program. You must always precede the file
name with D: (or Dl: if you need to specify the drive number), and
put the whole thing in quotation marks. If you forget the D:, the com-
puter will have no way to know that you want it to use the disk drive
rather dian some other device, such as die program recorder.

When you store a program on a diskette, you must always give it a
name, so that the computer can locate it when you want to retrieve it.
This was not necessary with the program recorder, since the computer
always loaded the first program it found on the tape.

If you SAVE a program with the same name as a file already on the
diskette, it will erase the existing file to write the new one. This is
helpful if you have made a minor change in a program you have
stored and want to replace die old version: just save the corrected pro-
gram on top of it. Of course, you must also be carefiil that you don't
erase an existing program by mistake: give your programs names that
are distinctive enough that you will never repeat them accidentally.

When you later want to retrieve the program, type

LOAD "D:FILENAME.EXT"

The computer will find the file with the name you specify, then load it
into the memory. You can LIST or RUN this program, as if you had
just typed it in. As with cassettes, you can LOAD and RUN a pro-
gram in one step, with the command

RUN "D:FILENAME.EXT"

The SAVE, LOAD, and RUN commands for diskette storage are
die same as the cassette commands SAVE "C:", LOAD "C:", and
RUN "C:", which were covered at die end of Chapter 9. The C: and
D: within the quotation marks tell die computer whedier to use die
cassette recorder or the disk drive; odierwise die statements are identi-
cal. There are also LIST and ENTER commands for diskettes— read
pages 150-51 of Chapter 9 to find out how diese work. There are no
disk commands that correspond to CSAVE and CLOAD.

DUPLICATING DISKS — _

There is anodier useiul command on die DOS menu diat makes a
backup copy of an entire disk: J for DUPLICATE DISK.

THE DISK DRIVE 169

If you don't already have the DOS menu on your screen, type
DOS. Then type J and press RETURN. The computer will respond

DUP DISK-SOURCE.DEST DRIVES?

This line is asking you which drive you want to copy the information
from and which drive you want to copy it to. If, like most people, you
have only one drive, your answer will be 1 or Dl to both questions.
Type the following and press RETURN:

D1,D1

With only one disk drive, you must swap the original and duplicate
disks in and out of your drive. The computer will use its memory as a
storage buflFer. It first reads the information in fitim the original disk,
then stops to let you remove the original axiA insert the copy disk.
Then, it writes a copy on this second disk of the information it had
just read. If, as usually happens, it could not hold all of the original
disk's stored information at one time, the computer then asks you to
reinsert the original and then again the copy so that it can duplicate
more of the information. It will keep asking you to swap your disk-
ettes until it has transferred all of the original's information.

When you have finally finished with the disk-swapping, the "desti-
nation disk" contains an exact copy of all of the files stored on the
original diskette. You should use this DUPLICATE DISK feature fre-
quendy to make backup copies of your important diskettes. That way,
if anything should happen to the diskette you are using, you will not
lose your work. You can use the duplicate copy in plzice of the origi-
nal, and continue with your work.

If you have more than one disk drive, you can avoid much of this
disk-swapping. You simply put the original disk in one of your drives
and the duplicate disk in the other. The computer will transfer all of
the information directly from one disk to the other without having to
ask you to change disks.

This convenience is one of the primary reasons why some people
choose to add a second drive to their system. While it doesn't allow
you to do anything you can't do with a single drive, a second drive
does simplify many disk-management operations.

You set up the second drive just like the first, except that you

170

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

plug Its cable into the back of the first drive, rather than into the
PERIPHERAL hole on the computer. Use a pen to move the black
switch inside the hole on the back of the second drive.

You can use this second drive just like the first one; simply refer to
it by the name D2. You can SAVE, LOAD, and RUN programs on
this drive by using D2 as part of the file name:

SAVE "D2:FILENAME.EXT"

The only thing you cannot do with the second drive is start the system
up: the computer always looks for its DOS instructions on drive
number 1.

SUMMARY

For permanent storage, a disk drive is faster, more flexible, and more
reliable than a program recorder. You can record many diflferent pro-
grams on a sin^e diskette, and retrieve them by name. You can also
load commercial disk programs and run them in a matter of seconds.

Some commercial programs will load themselves direcdy without
any special command. With others, you may need to give a LOAD or
a RUN command.

To save or load your own programs, you will need to use Atari's
Disk Operating System (DOS) disk. When you turn on your com-
puter with this disk in the drive, the computer will automatically load
some extra commands to let you SAVE and LOAD programs direcdy
You can also type the DOS command to do more specialized disk
operations, such as formatting a blank diskette, deleting files, or dupli-
cating an entire diskette.

B CHAPTER ELEVEN

Afterword

In the course of this book, you have seen many of the things you
can do with your Atari computer. You have learned how to set it up
and how to use preprograinuned software. You have learned to give
commzinds of your own and to write programs that will control the
computer in various ways. Finzilly, in this third section, you have
found out about Atzui's Program Recorder and Disk Drive, which
you can connect to your system to store programs permainendy.

This is only the beginning. With these basic skills, you can go on to
explore mjiny other possibilities. You can connect a printer or other
equipment to your system, auid write complex programs that solve
important problems.

In these final pages, I will try to give you an idea of some of these
other possibilities. I czmnot cover everything, though. If you want
more detailed information, check some of the books in the biblio-
graphy, or try some experiments on your own.

OTHER ADD-ONS

Atari's three printers were briefly mentioned at the end of Chapter 1 .
They diflfer in many ways, but all three have the same function: they
take the information that is stored electronically in the computer and
display it on paper

A printer is usefiil for many reasons. You might wcint to make a
permanent listing of a program, so that you aire sure you can type it
hack in if you should lose it. If you are writing a long program, the
printer will let you read it all at once, rather than one screen at a
time. And of course, you will need a printer if you want to use your
computer as a word processor.

172 THE EASY GUIDE TO YOUR ATARI 600XL/800XL

You connect an Atari printer to the computer using the hole labeled
PERIPHERAL, just as you would connect a program recorder or
disk drive. If you want to use several peripherals at once, you can
connect them all in a chain, as described in the chapter on the disk
drive (see page 155).

With the printer connected and turned on, you can normally use
it without further trouble. Many financial and word-processing
programs will let you print tables and text directly, with a simple
commamd.

If you have a program of your own stored in your computer's
memory, you can print a copy of it out on paper To do this, use the
following variation on the standard LIST command:

LIST "R"

The computer will then type the program out, line by line, just as it
would have displayed it on your television screen. The "P:" in the
statement tells the computer to send the program listing to the printer
rather than to the screen.

You can use the printer to do many things other than simply listing
programs on paper. You can display messages or lists of numbers on
paper, rather dian on the screen. On the 1025 printer, you can ask the
printer to type in special ways or to change the spacing between let-
ters. With the 1020 printer/plotter, you can even draw certain types
of graphics.

Another useful accessory is a telephone modem. This is a device that
lets you connect your computer to a telephone line so that it can com-
municate electronically with another computer. With this, you can
send a program to a friend across the country, or receive information
from one of the new electronic news and information services, such as
The Source.

If all goes as planned. Atari's 1030 direa-connect modem will let you
do this very easily. This device converts your computer's electronic
signals into a warbling sound, which it then sends direcdy over your
telephone lines. The modem has a small hole where you can plug
in a standard telephone cable. With the help of a special program,
your computer can send information direcdy to another computer. It
can also receive messages from another computer and store them in
its memory.

AFTERWORD 173

Atari is planning a large line of other add-on devices to match its
new XL series of computers. Some of these may never materialize,
but they are worth looking out for.

One interesting product is the Touch Tablet, a special electronic pad
which lets you draw graphic designs direcdy, without any progrzim-
ming. You draw with a special pen on the tablet, zind the computer
can sense exactiy where you move. It can then paint with an imagi-
nary pjiintbrush on the screen, precisely following your movements
on the pad.

An even more interesting idea is the light pen, which Atari has been
developing for several years. This is an electronic rod that you can
point at your television screen. With a special circuit, your computer
can sense exacdy which spot you are pointing to, and can move a
cursor to that point or paint it a certain color. As you do this, it will
seem as if your light pen were actually painting lines on your televi-
sion screen.

Atari has also planned an Expansion System, a large box that will
zillow you to cormect many different exp£insions to your computer,
including devices not made by Atari.

At this writing, all of these aidd-ons are speculation. They have all
been officially announced by Atari, but some may never be sold.
Check with your dealer for more information and for detailed
descriptions.

OTHER DIRECTIONS

If you have been reading the programming sections of this book,
you have learned some of the ways you can control your computer
directly. You can create a program, use variables to perform calcula-
tions, and control the program's flow with loops and decisions. With
special graphics commands, you can paint pictures in various ways on
the screen.

While you have now covered all the fundamental concepts, there
are many other thing you can do. We have really only scratched the
surface.

Within the BASIC programming language, there are specialized fea-
tures that let you control your computer in other ways. You can, for

174

THE EASY GUIDE TO YOUR ATARI 600XL/800XL

example, use arrays that let you store an entire table of numbers in the
memory, rather than giving them each separate variable names. You
can save words or numbers d& data an. cassette or diskette. You can then
later retrieve this information and reuse it in other programs.

There are several other programming languages that you can use
with your Atari computer. One of the most popular is Atari PILOT, a
graphics language for children. In diis language, you give commands
to move a small turtk around the screen. As it moves, the turtle lays
down a trail, which your child can use to create interesting pictures.

With any progranmiing language, the computer must translate the
English-like commands into the specific codes that it can use. This
process is slow and inefficient, so some people bypass the process com-
pletely for complex programs that must run quickly. They do this with
machine language, which allows direct control of the computer.

With machine lamguage, you are actually giving commands at the
deepest level of the computer, and can exert direct control over its
operation. Some of the advanced graphics features of the Atari com-
puter can only be controlled with direct machine language com-
mands: the animated figures zind missiles of video games are an
example of this. Machine language also lets the computer run much
more quickly, since inefficient operations are reduced.

Unfortunately, machine language is very technical and difficult to
use, since it forces you to keep track of every single operation the
computer must do. You must know a lot about the internal workings
of the machine, and must be willing to do tedious programming.
Even professional programmers usually avoid machine language
unless there is a clear need for it.

Machine language is closely related to assembly language, which is
slighdy more understandable. While you must still give meticulous
commands for every operation you wzint the computer to do, you can
use labels and verbal commands, rather dian numerical codes. The
computer can translate assembly language direcdy into machine lan-
guage when it uses it.

Some books and magazines take a middle ground between BASIC
and machine language, by using two special BASIC statements:
PEEK and POKE. These let you read and store values direcdy in the
computer's memory, without giving them a variable name. In this
way, you can exert some types of direct control over the internal oper-
ation of the computer, which you could not do otherwise without

AFTERWORD

machine language. The process is, however, quite cumbersome and
hard to understand.

Whatever you do, I hope that you will continue your explorations
beyond the end of this book. There are many other books and
resources that can help you lezim even more than I have been able to
cover in these pages. Your main resource, however, is yourself. Be
creative with your computer, and play ziround with it. There are
mamy wonderful things you can do with your machine, with whatever
approach you choose to take.

175

B APPENDIX A

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Texts

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "The easy guide to your Atari 600XL/800XL"

See other formats